Enable Nullable Reference Types

src/MediatR/IMediator.cs

@@ -24,7 +24,7 @@ public interface IMediator
         /// <param name="request">Request object</param>
         /// <param name="cancellationToken">Optional cancellation token</param>
         /// <returns>A task that represents the send operation. The task result contains the type erased handler response</returns>
-        Task<object> Send(object request, CancellationToken cancellationToken = default);
+        Task<object?> Send(object request, CancellationToken cancellationToken = default);
 
         /// <summary>
         /// Asynchronously send a notification to multiple handlers

src/MediatR/IPipelineBehavior.cs

@@ -17,7 +17,7 @@ namespace MediatR
     /// </summary>
     /// <typeparam name="TRequest">Request type</typeparam>
     /// <typeparam name="TResponse">Response type</typeparam>
-    public interface IPipelineBehavior<in TRequest, TResponse>
+    public interface IPipelineBehavior<in TRequest, TResponse> where TRequest : notnull
     {
         /// <summary>
         /// Pipeline handler. Perform any additional behavior and await the <paramref name="next"/> delegate as necessary

src/MediatR/Internal/HandlersOrderer.cs

@@ -7,6 +7,7 @@ namespace MediatR.Internal
     internal static class HandlersOrderer
     {
         public static IList<object> Prioritize<TRequest>(IList<object> handlers, TRequest request)
+            where TRequest : notnull
         {
             if (handlers.Count < 2)
             {

src/MediatR/Internal/RequestHandlerWrapper.cs

@@ -8,7 +8,7 @@ namespace MediatR.Internal
 
     internal abstract class RequestHandlerBase
     {
-        public abstract Task<object> Handle(object request, CancellationToken cancellationToken,
+        public abstract Task<object?> Handle(object request, CancellationToken cancellationToken,
             ServiceFactory serviceFactory);
 
         protected static THandler GetHandler<THandler>(ServiceFactory factory)
@@ -42,7 +42,7 @@ public abstract Task<TResponse> Handle(IRequest<TResponse> request, Cancellation
     internal class RequestHandlerWrapperImpl<TRequest, TResponse> : RequestHandlerWrapper<TResponse>
         where TRequest : IRequest<TResponse>
     {
-        public override Task<object> Handle(object request, CancellationToken cancellationToken,
+        public override Task<object?> Handle(object request, CancellationToken cancellationToken,
             ServiceFactory serviceFactory)
         {
             return Handle((IRequest<TResponse>)request, cancellationToken, serviceFactory)
@@ -52,7 +52,7 @@ public override Task<object> Handle(object request, CancellationToken cancellati
                     {
                         ExceptionDispatchInfo.Capture(t.Exception.InnerException).Throw();
                     }
-                    return (object)t.Result;
+                    return (object?)t.Result;
                 }, cancellationToken);
         }
 

src/MediatR/MediatR.csproj

@@ -3,7 +3,9 @@
   <PropertyGroup>
     <Description>Simple, unambitious mediator implementation in .NET</Description>
     <Copyright>Copyright Jimmy Bogard</Copyright>
-    <TargetFrameworks>net461;netstandard2.0</TargetFrameworks>
+    <TargetFrameworks>net461;netstandard2.0;netstandard2.1</TargetFrameworks>
+    <LangVersion>8</LangVersion>
+    <Nullable>enable</Nullable>
     <AssemblyName>MediatR</AssemblyName>
     <PackageId>MediatR</PackageId>
     <PackageTags>mediator;request;response;queries;commands;notifications</PackageTags>

src/MediatR/Mediator.cs

@@ -41,7 +41,7 @@ public Task<TResponse> Send<TResponse>(IRequest<TResponse> request, Cancellation
             return handler.Handle(request, cancellationToken, _serviceFactory);
         }
 
-        public Task<object> Send(object request, CancellationToken cancellationToken = default)
+        public Task<object?> Send(object request, CancellationToken cancellationToken = default)
         {
             if (request == null)
             {
@@ -58,7 +58,7 @@ public Task<object> Send(object request, CancellationToken cancellationToken = d
                 throw new ArgumentException($"{nameof(request)} does not implement ${nameof(IRequest)}");
             }
 
-            var responseType = requestInterfaceType.GetGenericArguments()[0];
+            var responseType = requestInterfaceType!.GetGenericArguments()[0];
             var handler = _requestHandlers.GetOrAdd(requestType,
                 t => Activator.CreateInstance(typeof(RequestHandlerWrapperImpl<,>).MakeGenericType(requestType, responseType)));
 

src/MediatR/NullableAttributes.cs

@@ -0,0 +1,140 @@
+#pragma warning disable MA0048 // File name must match type name
+#define INTERNAL_NULLABLE_ATTRIBUTES
+#if NETSTANDARD2_0 ||  NETCOREAPP2_0 ||  NETCOREAPP2_1 ||  NETCOREAPP2_2 || NET45 || NET451 || NET452 || NET6 || NET461 || NET462 || NET47 || NET471 || NET472 || NET48
+
+// https://github.com/dotnet/corefx/blob/48363ac826ccf66fbe31a5dcb1dc2aab9a7dd768/src/Common/src/CoreLib/System/Diagnostics/CodeAnalysis/NullableAttributes.cs
+
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+namespace System.Diagnostics.CodeAnalysis
+{
+    /// <summary>Specifies that null is allowed as an input even if the corresponding type disallows it.</summary>
+    [AttributeUsage(AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.Property, Inherited = false)]
+#if INTERNAL_NULLABLE_ATTRIBUTES
+    internal
+#else
+    public
+#endif
+        sealed class AllowNullAttribute : Attribute
+    { }
+
+    /// <summary>Specifies that null is disallowed as an input even if the corresponding type allows it.</summary>
+    [AttributeUsage(AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.Property, Inherited = false)]
+#if INTERNAL_NULLABLE_ATTRIBUTES
+    internal
+#else
+    public
+#endif
+        sealed class DisallowNullAttribute : Attribute
+    { }
+
+    /// <summary>Specifies that an output may be null even if the corresponding type disallows it.</summary>
+    [AttributeUsage(AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.Property | AttributeTargets.ReturnValue, Inherited = false)]
+#if INTERNAL_NULLABLE_ATTRIBUTES
+    internal
+#else
+    public
+#endif
+        sealed class MaybeNullAttribute : Attribute
+    { }
+
+    /// <summary>Specifies that an output will not be null even if the corresponding type allows it.</summary>
+    [AttributeUsage(AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.Property | AttributeTargets.ReturnValue, Inherited = false)]
+#if INTERNAL_NULLABLE_ATTRIBUTES
+    internal
+#else
+    public
+#endif
+        sealed class NotNullAttribute : Attribute
+    { }
+
+    /// <summary>Specifies that when a method returns <see cref="ReturnValue"/>, the parameter may be null even if the corresponding type disallows it.</summary>
+    [AttributeUsage(AttributeTargets.Parameter, Inherited = false)]
+#if INTERNAL_NULLABLE_ATTRIBUTES
+    internal
+#else
+    public
+#endif
+        sealed class MaybeNullWhenAttribute : Attribute
+    {
+        /// <summary>Initializes the attribute with the specified return value condition.</summary>
+        /// <param name="returnValue">
+        /// The return value condition. If the method returns this value, the associated parameter may be null.
+        /// </param>
+        public MaybeNullWhenAttribute(bool returnValue) => ReturnValue = returnValue;
+
+        /// <summary>Gets the return value condition.</summary>
+        public bool ReturnValue { get; }
+    }
+
+    /// <summary>Specifies that when a method returns <see cref="ReturnValue"/>, the parameter will not be null even if the corresponding type allows it.</summary>
+    [AttributeUsage(AttributeTargets.Parameter, Inherited = false)]
+#if INTERNAL_NULLABLE_ATTRIBUTES
+    internal
+#else
+    public
+#endif
+        sealed class NotNullWhenAttribute : Attribute
+    {
+        /// <summary>Initializes the attribute with the specified return value condition.</summary>
+        /// <param name="returnValue">
+        /// The return value condition. If the method returns this value, the associated parameter will not be null.
+        /// </param>
+        public NotNullWhenAttribute(bool returnValue) => ReturnValue = returnValue;
+
+        /// <summary>Gets the return value condition.</summary>
+        public bool ReturnValue { get; }
+    }
+
+    /// <summary>Specifies that the output will be non-null if the named parameter is non-null.</summary>
+    [AttributeUsage(AttributeTargets.Parameter | AttributeTargets.Property | AttributeTargets.ReturnValue, AllowMultiple = true, Inherited = false)]
+#if INTERNAL_NULLABLE_ATTRIBUTES
+    internal
+#else
+    public
+#endif
+        sealed class NotNullIfNotNullAttribute : Attribute
+    {
+        /// <summary>Initializes the attribute with the associated parameter name.</summary>
+        /// <param name="parameterName">
+        /// The associated parameter name.  The output will be non-null if the argument to the parameter specified is non-null.
+        /// </param>
+        public NotNullIfNotNullAttribute(string parameterName) => ParameterName = parameterName;
+
+        /// <summary>Gets the associated parameter name.</summary>
+        public string ParameterName { get; }
+    }
+
+    /// <summary>Applied to a method that will never return under any circumstance.</summary>
+    [AttributeUsage(AttributeTargets.Method, Inherited = false)]
+#if INTERNAL_NULLABLE_ATTRIBUTES
+    internal
+#else
+    public
+#endif
+        sealed class DoesNotReturnAttribute : Attribute
+    { }
+
+    /// <summary>Specifies that the method will not return if the associated Boolean parameter is passed the specified value.</summary>
+    [AttributeUsage(AttributeTargets.Parameter, Inherited = false)]
+#if INTERNAL_NULLABLE_ATTRIBUTES
+    internal
+#else
+    public
+#endif
+        sealed class DoesNotReturnIfAttribute : Attribute
+    {
+        /// <summary>Initializes the attribute with the specified parameter value.</summary>
+        /// <param name="parameterValue">
+        /// The condition parameter value. Code after the method will be considered unreachable by diagnostics if the argument to
+        /// the associated parameter matches this value.
+        /// </param>
+        public DoesNotReturnIfAttribute(bool parameterValue) => ParameterValue = parameterValue;
+
+        /// <summary>Gets the condition parameter value.</summary>
+        public bool ParameterValue { get; }
+    }
+}
+#endif

src/MediatR/Pipeline/IRequestExceptionAction.cs

@@ -10,6 +10,7 @@ namespace MediatR.Pipeline
     /// <typeparam name="TRequest">Request type</typeparam>
     /// <typeparam name="TException">Exception type</typeparam>
     public interface IRequestExceptionAction<in TRequest, in TException>
+        where TRequest : notnull
         where TException : Exception
     {
         /// <summary>
@@ -30,6 +31,7 @@ public interface IRequestExceptionAction<in TRequest, in TException>
     /// </summary>
     /// <typeparam name="TRequest">The type of failed request</typeparam>
     public interface IRequestExceptionAction<in TRequest> : IRequestExceptionAction<TRequest, Exception>
+        where TRequest : notnull
     {
     }
 
@@ -60,6 +62,7 @@ async Task IRequestExceptionAction<TRequest, Exception>.Execute(TRequest request
     /// <typeparam name="TRequest">Request type</typeparam>
     /// <typeparam name="TException">Exception type</typeparam>
     public abstract class RequestExceptionAction<TRequest, TException> : IRequestExceptionAction<TRequest, TException>
+        where TRequest : notnull
         where TException : Exception
     {
         Task IRequestExceptionAction<TRequest, TException>.Execute(TRequest request, TException exception, CancellationToken cancellationToken)
@@ -81,6 +84,7 @@ Task IRequestExceptionAction<TRequest, TException>.Execute(TRequest request, TEx
     /// </summary>
     /// <typeparam name="TRequest">Request type</typeparam>
     public abstract class RequestExceptionAction<TRequest> : IRequestExceptionAction<TRequest>
+        where TRequest : notnull
     {
         Task IRequestExceptionAction<TRequest, Exception>.Execute(TRequest request, Exception exception, CancellationToken cancellationToken)
         {

src/MediatR/Pipeline/IRequestExceptionHandler.cs

@@ -11,6 +11,7 @@ namespace MediatR.Pipeline
     /// <typeparam name="TResponse">Response type</typeparam>
     /// <typeparam name="TException">Exception type</typeparam>
     public interface IRequestExceptionHandler<in TRequest, TResponse, TException>
+        where TRequest : notnull
         where TException : Exception
     {
         /// <summary>
@@ -30,6 +31,7 @@ public interface IRequestExceptionHandler<in TRequest, TResponse, TException>
     /// <typeparam name="TRequest">Request type</typeparam>
     /// <typeparam name="TResponse">Response type</typeparam>
     public interface IRequestExceptionHandler<in TRequest, TResponse> : IRequestExceptionHandler<TRequest, TResponse, Exception>
+        where TRequest : notnull
     {
     }
 
@@ -39,6 +41,7 @@ public interface IRequestExceptionHandler<in TRequest, TResponse> : IRequestExce
     /// <typeparam name="TRequest">Request type</typeparam>
     /// <typeparam name="TResponse">Response type</typeparam>
     public abstract class AsyncRequestExceptionHandler<TRequest, TResponse> : IRequestExceptionHandler<TRequest, TResponse>
+        where TRequest : notnull
     {
         async Task IRequestExceptionHandler<TRequest, TResponse, Exception>.Handle(TRequest request, Exception exception, RequestExceptionHandlerState<TResponse> state, CancellationToken cancellationToken)
         {
@@ -62,6 +65,7 @@ async Task IRequestExceptionHandler<TRequest, TResponse, Exception>.Handle(TRequ
     /// <typeparam name="TResponse">Response type</typeparam>
     /// <typeparam name="TException">Exception type</typeparam>
     public abstract class RequestExceptionHandler<TRequest, TResponse, TException> : IRequestExceptionHandler<TRequest, TResponse, TException>
+        where TRequest : notnull
         where TException : Exception
     {
         Task IRequestExceptionHandler<TRequest, TResponse, TException>.Handle(TRequest request, TException exception, RequestExceptionHandlerState<TResponse> state, CancellationToken cancellationToken)
@@ -85,6 +89,7 @@ Task IRequestExceptionHandler<TRequest, TResponse, TException>.Handle(TRequest r
     /// <typeparam name="TRequest">Request type</typeparam>
     /// <typeparam name="TResponse">Response type</typeparam>
     public abstract class RequestExceptionHandler<TRequest, TResponse> : IRequestExceptionHandler<TRequest, TResponse>
+        where TRequest : notnull
     {
         Task IRequestExceptionHandler<TRequest, TResponse, Exception>.Handle(TRequest request, Exception exception, RequestExceptionHandlerState<TResponse> state, CancellationToken cancellationToken)
         {

src/MediatR/Pipeline/IRequestPostProcessor.cs

@@ -8,7 +8,7 @@ namespace MediatR.Pipeline
     /// </summary>
     /// <typeparam name="TRequest">Request type</typeparam>
     /// <typeparam name="TResponse">Response type</typeparam>
-    public interface IRequestPostProcessor<in TRequest, in TResponse>
+    public interface IRequestPostProcessor<in TRequest, in TResponse> where TRequest : notnull
     {
         /// <summary>
         /// Process method executes after the Handle method on your handler

src/MediatR/Pipeline/IRequestPreProcessor.cs

@@ -7,7 +7,7 @@ namespace MediatR.Pipeline
     /// Defined a request pre-processor for a handler
     /// </summary>
     /// <typeparam name="TRequest">Request type</typeparam>
-    public interface IRequestPreProcessor<in TRequest>
+    public interface IRequestPreProcessor<in TRequest> where TRequest : notnull
     {
         /// <summary>
         /// Process method executes before calling the Handle method on your handler

src/MediatR/Pipeline/RequestExceptionActionProcessorBehavior.cs

@@ -16,6 +16,7 @@ namespace MediatR.Pipeline
     /// <typeparam name="TRequest">Request type</typeparam>
     /// <typeparam name="TResponse">Response type</typeparam>
     public class RequestExceptionActionProcessorBehavior<TRequest, TResponse> : IPipelineBehavior<TRequest, TResponse>
+        where TRequest : notnull
     {
         private readonly ServiceFactory _serviceFactory;
 

src/MediatR/Pipeline/RequestExceptionHandlerState.cs

@@ -1,3 +1,5 @@
+using System.Diagnostics.CodeAnalysis;
+
 namespace MediatR.Pipeline
 {
     /// <summary>
@@ -14,13 +16,14 @@ public class RequestExceptionHandlerState<TResponse>
         /// <summary>
         /// The response that is returned if <see cref="Handled"/> is  <code>true</code>.
         /// </summary>
-        public TResponse Response { get; private set; }
+        [MaybeNull]
+        public TResponse Response { get; private set; } = default!;
 
         /// <summary>
         /// Call to indicate whether the current exception should be considered handled and the specified response should be returned.
         /// </summary>
         /// <param name="response">Set the response that will be returned.</param>
-        public void SetHandled(TResponse response = default)
+        public void SetHandled(TResponse response)
         {
             Handled = true;
             Response = response;

src/MediatR/Pipeline/RequestExceptionProcessorBehavior.cs

@@ -16,6 +16,7 @@ namespace MediatR.Pipeline
     /// <typeparam name="TRequest">Request type</typeparam>
     /// <typeparam name="TResponse">Response type</typeparam>
     public class RequestExceptionProcessorBehavior<TRequest, TResponse> : IPipelineBehavior<TRequest, TResponse>
+        where TRequest : notnull
     {
         private readonly ServiceFactory _serviceFactory;
 
@@ -30,7 +31,7 @@ public async Task<TResponse> Handle(TRequest request, CancellationToken cancella
             catch (Exception exception)
             {
                 var state = new RequestExceptionHandlerState<TResponse>();
-                Type exceptionType = null;
+                Type? exceptionType = null;
 
                 while (!state.Handled && exceptionType != typeof(Exception))
                 {
@@ -53,7 +54,7 @@ public async Task<TResponse> Handle(TRequest request, CancellationToken cancella
                     throw;
                 }
 
-                return state.Response;
+                return state.Response!; //cannot be null if Handled
             }
         }
 

src/MediatR/Pipeline/RequestPostProcessorBehavior.cs

@@ -10,6 +10,7 @@ namespace MediatR.Pipeline
     /// <typeparam name="TRequest">Request type</typeparam>
     /// <typeparam name="TResponse">Response type</typeparam>
     public class RequestPostProcessorBehavior<TRequest, TResponse> : IPipelineBehavior<TRequest, TResponse>
+        where TRequest : notnull
     {
         private readonly IEnumerable<IRequestPostProcessor<TRequest, TResponse>> _postProcessors;
 

src/MediatR/Pipeline/RequestPreProcessorBehavior.cs

@@ -10,6 +10,7 @@ namespace MediatR.Pipeline
     /// <typeparam name="TRequest"></typeparam>
     /// <typeparam name="TResponse"></typeparam>
     public class RequestPreProcessorBehavior<TRequest, TResponse> : IPipelineBehavior<TRequest, TResponse>
+        where TRequest : notnull
     {
         private readonly IEnumerable<IRequestPreProcessor<TRequest>> _preProcessors;
 

test/MediatR.Tests/MediatR.Tests.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks>net461;netcoreapp2.1</TargetFrameworks>
+    <TargetFrameworks>net461;netcoreapp2.1;netcoreapp3.1</TargetFrameworks>
     <AssemblyName>MediatR.Tests</AssemblyName>
     <PackageId>MediatR.Tests</PackageId>
   </PropertyGroup>