Optimize empty string compares (#16435)

* use String.IsNullOrEmpty() or .Length = 0 for checks of empty strings

* To be on the safe side and for consistency reasons: Always use String.IsNullOrEmpty

src/Compiler/AbstractIL/il.fs

@@ -805,7 +805,7 @@ type ILTypeRef =
     member tref.AddQualifiedNameExtension basic =
         let sco = tref.Scope.QualifiedName
 
-        if sco = "" then
+        if String.IsNullOrEmpty(sco) then
             basic
         else
             String.concat ", " [ basic; sco ]

src/Compiler/AbstractIL/ilwrite.fs

@@ -652,7 +652,7 @@ let GetBytesAsBlobIdx cenv (bytes: byte[]) =
     else cenv.blobs.FindOrAddSharedEntry bytes
 
 let GetStringHeapIdx cenv s =
-    if s = "" then 0
+    if String.IsNullOrEmpty(s) then 0
     else cenv.strings.FindOrAddSharedEntry s
 
 let GetGuidIdx cenv info = cenv.guids.FindOrAddSharedEntry info

src/Compiler/Checking/CheckDeclarations.fs

@@ -2495,7 +2495,7 @@ module TcExceptionDeclarations =
 
     let TcExnSignature (cenv: cenv) envInitial parent tpenv (SynExceptionSig(exnRepr=core; members=aug), scopem) =
         match core with
-        | SynExceptionDefnRepr(caseName = SynUnionCase(ident = SynIdent(ident, _))) when ident.idText = "" ->
+        | SynExceptionDefnRepr(caseName = SynUnionCase(ident = SynIdent(ident, _))) when String.IsNullOrEmpty(ident.idText) ->
             [], [], None, envInitial
         | _ ->
             let g = cenv.g
@@ -4306,7 +4306,7 @@ module TcDeclarations =
         // Convert auto properties to let bindings in the pre-list
         let rec preAutoProps memb =
             match memb with 
-            | SynMemberDefn.AutoProperty(ident = id) when id.idText = "" -> []
+            | SynMemberDefn.AutoProperty(ident = id) when String.IsNullOrEmpty(id.idText) -> []
             | SynMemberDefn.AutoProperty(attributes=Attributes attribs; isStatic=isStatic; ident=id; typeOpt=tyOpt; propKind=propKind; xmlDoc=xmlDoc; synExpr=synExpr; range=mWholeAutoProp) -> 
                 // Only the keep the field-targeted attributes
                 let attribs = attribs |> List.filter (fun a -> match a.Target with Some t when t.idText = "field" -> true | _ -> false)
@@ -4334,7 +4334,7 @@ module TcDeclarations =
         // Convert auto properties to member bindings in the post-list
         let rec postAutoProps memb =
             match memb with 
-            | SynMemberDefn.AutoProperty(ident = id) when id.idText = "" -> []
+            | SynMemberDefn.AutoProperty(ident = id) when String.IsNullOrEmpty(id.idText) -> []
             | SynMemberDefn.AutoProperty(attributes=Attributes attribs; isStatic=isStatic; ident=id; typeOpt=tyOpt; propKind=propKind; memberFlags=memberFlags; memberFlagsForSet=memberFlagsForSet; xmlDoc=xmlDoc; accessibility=access; trivia = { GetSetKeywords = mGetSetOpt }) ->
                 let mMemberPortion = id.idRange
                 // Only the keep the non-field-targeted attributes
@@ -5073,7 +5073,7 @@ let rec TcModuleOrNamespaceElementNonMutRec (cenv: cenv) parent typeNames scopem
           return ([], [], []), env, env
 
       | SynModuleDecl.Exception (SynExceptionDefn(SynExceptionDefnRepr(caseName = SynUnionCase(ident = SynIdent(id, _))) as exnRepr, withKeyword, ms, mExDefn), m) ->
-          if id.idText = "" then
+          if String.IsNullOrEmpty(id.idText) then
               return ([], [], []), env, env
           else
               let edef = SynExceptionDefn(exnRepr, withKeyword, desugarGetSetMembers ms, mExDefn)

src/Compiler/Checking/CheckFormatStrings.fs

@@ -2,6 +2,7 @@
 
 module internal FSharp.Compiler.CheckFormatStrings
 
+open System
 open System.Text
 open Internal.Utilities.Library
 open Internal.Utilities.Library.Extras
@@ -375,7 +376,7 @@ let parseFormatStringInternal
                         failwith (FSComp.SR.forFormatInvalidForInterpolated3())
                     else
                         let dotnetAlignment = match widthValue with None -> "" | Some w -> "," + (if info.leftJustify then "-" else "") + string w
-                        let dotnetNumberFormat = match fmt[i+1..i2-1] with "" -> "" | s -> ":" + s
+                        let dotnetNumberFormat = match fmt[i+1..i2-1] with s when String.IsNullOrEmpty(s) -> "" | s -> ":" + s
                         appendToDotnetFormatString ("{" + string dotnetFormatStringInterpolationHoleCount + dotnetAlignment  + dotnetNumberFormat + "}")
                         dotnetFormatStringInterpolationHoleCount <- dotnetFormatStringInterpolationHoleCount + 1
                         i2+1

src/Compiler/Checking/InfoReader.fs

@@ -4,6 +4,7 @@
 /// Select members from a type by name, searching the type hierarchy if needed
 module internal FSharp.Compiler.InfoReader
 
+open System
 open System.Collections.Concurrent
 open System.Collections.Generic
 open Internal.Utilities.Library
@@ -1097,14 +1098,14 @@ let GetXmlDocSigOfEntityRef infoReader m (eref: EntityRef) =
     else
         let ccuFileName = libFileOfEntityRef eref
         let m = eref.Deref
-        if m.XmlDocSig = "" then
+        if String.IsNullOrEmpty(m.XmlDocSig) then
             m.XmlDocSig <- XmlDocSigOfEntity eref
         Some (ccuFileName, m.XmlDocSig)
 
 let GetXmlDocSigOfScopedValRef g (tcref: TyconRef) (vref: ValRef) = 
     let ccuFileName = libFileOfEntityRef tcref
     let v = vref.Deref
-    if v.XmlDocSig = "" && v.HasDeclaringEntity then
+    if String.IsNullOrEmpty(v.XmlDocSig) && v.HasDeclaringEntity then
         let ap = buildAccessPath vref.DeclaringEntity.CompilationPathOpt
         let path =
             if vref.DeclaringEntity.IsModule then
@@ -1118,14 +1119,14 @@ let GetXmlDocSigOfScopedValRef g (tcref: TyconRef) (vref: ValRef) =
 let GetXmlDocSigOfRecdFieldRef (rfref: RecdFieldRef) = 
     let tcref = rfref.TyconRef
     let ccuFileName = libFileOfEntityRef tcref 
-    if rfref.RecdField.XmlDocSig = "" then
+    if String.IsNullOrEmpty(rfref.RecdField.XmlDocSig) then
         rfref.RecdField.XmlDocSig <- XmlDocSigOfProperty [tcref.CompiledRepresentationForNamedType.FullName; rfref.RecdField.LogicalName]
     Some (ccuFileName, rfref.RecdField.XmlDocSig)
 
 let GetXmlDocSigOfUnionCaseRef (ucref: UnionCaseRef) = 
     let tcref =  ucref.TyconRef
     let ccuFileName = libFileOfEntityRef tcref
-    if  ucref.UnionCase.XmlDocSig = "" then
+    if String.IsNullOrEmpty(ucref.UnionCase.XmlDocSig) then
         ucref.UnionCase.XmlDocSig <- XmlDocSigOfUnionCase [tcref.CompiledRepresentationForNamedType.FullName; ucref.CaseName]
     Some (ccuFileName, ucref.UnionCase.XmlDocSig)
 
@@ -1171,7 +1172,7 @@ let GetXmlDocSigOfValRef g (vref: ValRef) =
     if not vref.IsLocalRef then
         let ccuFileName = vref.nlr.Ccu.FileName
         let v = vref.Deref
-        if v.XmlDocSig = "" && v.HasDeclaringEntity then
+        if String.IsNullOrEmpty(v.XmlDocSig) && v.HasDeclaringEntity then
             v.XmlDocSig <- XmlDocSigOfVal g false vref.DeclaringEntity.CompiledRepresentationForNamedType.Name v
         Some (ccuFileName, v.XmlDocSig)
     else 

src/Compiler/Checking/NicePrint.fs

@@ -224,7 +224,7 @@ module internal PrintUtilities =
                         else s)
 
             let pathText = trimPathByDisplayEnv denv path
-            if pathText = "" then tyconTextL else leftL (tagUnknownEntity pathText) ^^ tyconTextL
+            if String.IsNullOrEmpty(pathText) then tyconTextL else leftL (tagUnknownEntity pathText) ^^ tyconTextL
 
     let layoutBuiltinAttribute (denv: DisplayEnv) (attrib: BuiltinAttribInfo) =
         let tcref = attrib.TyconRef
@@ -2793,7 +2793,7 @@ let minimalStringsOfTwoTypes denv ty1 ty2 =
         let denv = denv.SetOpenPaths []
         let denv = { denv with includeStaticParametersInTypeNames=true }
         let makeName t =
-            let assemblyName = PrintTypes.layoutAssemblyName denv t |> function | null | "" -> "" | name -> sprintf " (%s)" name
+            let assemblyName = PrintTypes.layoutAssemblyName denv t |> fun name -> if String.IsNullOrEmpty(name) then "" else sprintf " (%s)" name
             sprintf "%s%s" (stringOfTy denv t) assemblyName
 
         (makeName ty1, makeName ty2, stringOfTyparConstraints denv tpcs)

src/Compiler/CodeGen/IlxGen.fs

@@ -5,6 +5,7 @@ module internal FSharp.Compiler.IlxGen
 
 open FSharp.Compiler.IlxGenSupport
 
+open System
 open System.IO
 open System.Reflection
 open System.Collections.Generic
@@ -420,7 +421,7 @@ let CompLocForFixedPath fragName qname (CompPath(sref, cpath)) =
     let ns = List.map fst ns
     let ns = textOfPath ns
     let encl = t |> List.map (fun (s, _) -> s)
-    let ns = if ns = "" then None else Some ns
+    let ns = if String.IsNullOrEmpty(ns) then None else Some ns
 
     {
         QualifiedNameOfFile = fragName
@@ -2833,7 +2834,7 @@ and GenExprPreSteps (cenv: cenv) (cgbuf: CodeGenBuffer) eenv expr sequel =
     match expr with
     | Expr.Sequential((DebugPointExpr g debugPointName) as dpExpr, codeExpr, NormalSeq, m) ->
         match cenv.namedDebugPointsForInlinedCode.TryGetValue({ Range = m; Name = debugPointName }) with
-        | false, _ when debugPointName = "" -> CG.EmitDebugPoint cgbuf m
+        | false, _ when String.IsNullOrEmpty(debugPointName) -> CG.EmitDebugPoint cgbuf m
         | false, _ ->
             // printfn $"---- Unfound debug point {debugPointName} at {m}"
             // for KeyValue(k,v) in cenv.namedDebugPointsForInlinedCode do
@@ -8614,7 +8615,7 @@ and GenMarshal cenv attribs =
                 let safeArrayUserDefinedSubType =
                     // the argument is a System.Type obj, but it's written to MD as a UTF8 string
                     match decoder.FindTypeName "SafeArrayUserDefinedSubType" "" with
-                    | "" -> None
+                    | x when String.IsNullOrEmpty(x) -> None
                     | res ->
                         if
                             (safeArraySubType = ILNativeVariant.IDispatch)

src/Compiler/Driver/CompilerDiagnostics.fs

@@ -896,9 +896,11 @@ type Exception with
                     [ knownReturnType; genericParametersMessage; argsMessage ]
                     |> List.choose id
                     |> String.concat (nl + nl)
-                    |> function
-                        | "" -> nl
-                        | result -> nl + nl + result + nl + nl
+                    |> fun result ->
+                        if String.IsNullOrEmpty(result) then
+                            nl
+                        else
+                            nl + nl + result + nl + nl
 
                 match failure with
                 | NoOverloadsFound(methodName, overloads, _) ->

src/Compiler/Driver/CompilerOptions.fs

@@ -109,7 +109,7 @@ let compilerOptionUsage (CompilerOption(s, tag, spec, _, _)) =
     | OptionFloat _ -> sprintf "--%s:%s" s tag
     | OptionRest _ -> sprintf "--%s ..." s
     | OptionGeneral _ ->
-        if tag = "" then
+        if String.IsNullOrEmpty(tag) then
             sprintf "%s" s
         else
             sprintf "%s:%s" s tag (* still being decided *)
@@ -270,7 +270,7 @@ let ParseCompilerOptions (collectOtherArgument: string -> unit, blocks: Compiler
         let optArgs = String.Join(":", opts[1..])
 
         let opt =
-            if option = "" then
+            if String.IsNullOrEmpty(option) then
                 ""
             // if it doesn't start with a '-' or '/', reject outright
             elif option[0] <> '-' && option[0] <> '/' then
@@ -298,13 +298,13 @@ let ParseCompilerOptions (collectOtherArgument: string -> unit, blocks: Compiler
         opt, token, optArgs
 
     let getOptionArg compilerOption (argString: string) =
-        if argString = "" then
+        if String.IsNullOrEmpty(argString) then
             errorR (Error(FSComp.SR.buildOptionRequiresParameter (compilerOptionUsage compilerOption), rangeCmdArgs))
 
         argString
 
     let getOptionArgList compilerOption (argString: string) =
-        if argString = "" then
+        if String.IsNullOrEmpty(argString) then
             errorR (Error(FSComp.SR.buildOptionRequiresParameter (compilerOptionUsage compilerOption), rangeCmdArgs))
             []
         else
@@ -375,19 +375,19 @@ let ParseCompilerOptions (collectOtherArgument: string -> unit, blocks: Compiler
                     reportDeprecatedOption d
                     f blocks
                     t
-                | CompilerOption(s, _, OptionUnit f, d, _) :: _ when optToken = s && argString = "" ->
+                | CompilerOption(s, _, OptionUnit f, d, _) :: _ when optToken = s && String.IsNullOrEmpty(argString) ->
                     reportDeprecatedOption d
                     f ()
                     t
-                | CompilerOption(s, _, OptionSwitch f, d, _) :: _ when getSwitchOpt optToken = s && argString = "" ->
+                | CompilerOption(s, _, OptionSwitch f, d, _) :: _ when getSwitchOpt optToken = s && String.IsNullOrEmpty(argString) ->
                     reportDeprecatedOption d
                     f (getSwitch opt)
                     t
-                | CompilerOption(s, _, OptionSet f, d, _) :: _ when optToken = s && argString = "" ->
+                | CompilerOption(s, _, OptionSet f, d, _) :: _ when optToken = s && String.IsNullOrEmpty(argString) ->
                     reportDeprecatedOption d
                     f.Value <- true
                     t
-                | CompilerOption(s, _, OptionClear f, d, _) :: _ when optToken = s && argString = "" ->
+                | CompilerOption(s, _, OptionClear f, d, _) :: _ when optToken = s && String.IsNullOrEmpty(argString) ->
                     reportDeprecatedOption d
                     f.Value <- false
                     t

src/Compiler/Driver/CreateILModule.fs

@@ -584,19 +584,22 @@ module MainModuleBuilder =
                 [ resource ]
 
         // a user cannot specify both win32res and win32manifest
-        if not (tcConfig.win32manifest = "") && not (tcConfig.win32res = "") then
+        if
+            not (String.IsNullOrEmpty(tcConfig.win32manifest))
+            && not (String.IsNullOrEmpty(tcConfig.win32res))
+        then
             error (Error(FSComp.SR.fscTwoResourceManifests (), rangeCmdArgs))
 
         let win32Manifest =
             // use custom manifest if provided
-            if not (tcConfig.win32manifest = "") then
+            if not (String.IsNullOrEmpty(tcConfig.win32manifest)) then
                 tcConfig.win32manifest
 
             // don't embed a manifest if target is not an exe, if manifest is specifically excluded, if another native resource is being included, or if running on mono
             elif
                 not (tcConfig.target.IsExe)
                 || not (tcConfig.includewin32manifest)
-                || not (tcConfig.win32res = "")
+                || not (String.IsNullOrEmpty(tcConfig.win32res))
             then
                 ""
             // otherwise, include the default manifest
@@ -618,9 +621,9 @@ module MainModuleBuilder =
             [
                 for av in assemblyVersionResources assemblyVersion do
                     ILNativeResource.Out av
-                if not (tcConfig.win32res = "") then
+                if not (String.IsNullOrEmpty(tcConfig.win32res)) then
                     ILNativeResource.Out(FileSystem.OpenFileForReadShim(tcConfig.win32res).ReadAllBytes())
-                if tcConfig.includewin32manifest && not (win32Manifest = "") then
+                if tcConfig.includewin32manifest && not (String.IsNullOrEmpty(win32Manifest)) then
                     ILNativeResource.Out
                         [|
                             yield! ResFileFormat.ResFileHeader()
@@ -631,8 +634,8 @@ module MainModuleBuilder =
                                 ))
                         |]
                 if
-                    tcConfig.win32res = ""
-                    && tcConfig.win32icon <> ""
+                    String.IsNullOrEmpty(tcConfig.win32res)
+                    && not (String.IsNullOrEmpty(tcConfig.win32icon))
                     && tcConfig.target <> CompilerTarget.Dll
                 then
                     use ms = new MemoryStream()

src/Compiler/Driver/fsc.fs

@@ -349,7 +349,7 @@ module InterfaceFileWriter =
         let writeAllToSameFile declaredImpls =
             /// Use a UTF-8 Encoding with no Byte Order Mark
             let os =
-                if tcConfig.printSignatureFile = "" then
+                if String.IsNullOrEmpty(tcConfig.printSignatureFile) then
                     Console.Out
                 else
                     FileSystem

src/Compiler/Facilities/CompilerLocation.fs

@@ -28,8 +28,7 @@ module internal FSharpEnvironment =
     let FSharpCoreLibRunningVersion =
         try
             match versionOf<Unit> with
-            | null -> None
-            | "" -> None
+            | s when String.IsNullOrEmpty(s) -> None
             | s -> Some(s)
         with _ ->
             None
@@ -184,7 +183,7 @@ module internal FSharpEnvironment =
                 | None -> ()
                 | Some(p: string) ->
                     match Path.GetDirectoryName(p) with
-                    | s when s = "" || isNull s || Path.GetFileName(p) = "packages" || s = p -> ()
+                    | s when String.IsNullOrEmpty(s) || Path.GetFileName(p) = "packages" || s = p -> ()
                     | parentDir -> yield! searchParentDirChain (Some parentDir) assemblyName
 
                 for p in searchToolPaths path compilerToolPaths do

src/Compiler/Interactive/fsi.fs

@@ -933,12 +933,12 @@ type DiagnosticsLogger with
 
 /// Get the directory name from a string, with some defaults if it doesn't have one
 let internal directoryName (s: string) =
-    if s = "" then
+    if String.IsNullOrEmpty(s) then
         "."
     else
         match Path.GetDirectoryName s with
         | null -> if FileSystem.IsPathRootedShim s then s else "."
-        | res -> if res = "" then "." else res
+        | res -> if String.IsNullOrEmpty(res) then "." else res
 
 //----------------------------------------------------------------------------
 // cmd line - state for options

src/Compiler/Service/FSharpCheckerResults.fs

@@ -2252,7 +2252,7 @@ type internal TypeCheckInfo
                         | Some itemRange ->
                             let projectDir =
                                 FileSystem.GetDirectoryNameShim(
-                                    if projectFileName = "" then
+                                    if String.IsNullOrEmpty(projectFileName) then
                                         mainInputFileName
                                     else
                                         projectFileName

src/Compiler/Service/ServiceAnalysis.fs

@@ -2,6 +2,7 @@
 
 namespace FSharp.Compiler.EditorServices
 
+open System
 open System.Collections.Generic
 open System.Runtime.CompilerServices
 open Internal.Utilities.Library
@@ -332,7 +333,10 @@ module SimplifyNames =
                             - partialName.PartialIdent.Length
                             - (getPlidLength partialName.QualifyingIdents)
 
-                        if partialName.PartialIdent = "" || List.isEmpty partialName.QualifyingIdents then
+                        if
+                            String.IsNullOrEmpty(partialName.PartialIdent)
+                            || List.isEmpty partialName.QualifyingIdents
+                        then
                             None
                         else
                             Some(symbolUse, partialName.QualifyingIdents, plidStartCol, partialName.PartialIdent))