C#: Fix nullability warnings after .net 5 upgrade

Author: tamasvajk

csharp/extractor/Semmle.Extraction.CSharp/Entities/Accessor.cs

@@ -13,7 +13,7 @@ protected Accessor(Context cx, IMethodSymbol init)
         /// Gets the property symbol associated accessor `symbol`, or `null`
         /// if there is no associated symbol.
         /// </summary>
-        public static IPropertySymbol GetPropertySymbol(IMethodSymbol symbol)
+        public static IPropertySymbol? GetPropertySymbol(IMethodSymbol symbol)
         {
             // Usually, the property/indexer can be fetched from the associated symbol
             if (symbol.AssociatedSymbol is IPropertySymbol prop)
@@ -29,7 +29,7 @@ public static IPropertySymbol GetPropertySymbol(IMethodSymbol symbol)
         /// <summary>
         /// Gets the property symbol associated with this accessor.
         /// </summary>
-        private IPropertySymbol PropertySymbol => GetPropertySymbol(Symbol);
+        private IPropertySymbol? PropertySymbol => GetPropertySymbol(Symbol);
 
         public new Accessor OriginalDefinition => Create(Context, Symbol.OriginalDefinition);
 

csharp/extractor/Semmle.Extraction.CSharp/Entities/CachedSymbol.cs

@@ -101,7 +101,7 @@ protected void BindComments()
 
         protected virtual T BodyDeclaringSymbol => Symbol;
 
-        public BlockSyntax Block
+        public BlockSyntax? Block
         {
             get
             {
@@ -112,7 +112,7 @@ public BlockSyntax Block
             }
         }
 
-        public ExpressionSyntax ExpressionBody
+        public ExpressionSyntax? ExpressionBody
         {
             get
             {

csharp/extractor/Semmle.Extraction.CSharp/Entities/Constructor.cs

@@ -86,7 +86,7 @@ protected override void ExtractInitializers(TextWriter trapFile)
             }
         }
 
-        private ConstructorDeclarationSyntax Syntax
+        private ConstructorDeclarationSyntax? Syntax
         {
             get
             {
@@ -122,7 +122,7 @@ public override void WriteId(TextWriter trapFile)
             trapFile.Write(";constructor");
         }
 
-        private ConstructorDeclarationSyntax GetSyntax() =>
+        private ConstructorDeclarationSyntax? GetSyntax() =>
             Symbol.DeclaringSyntaxReferences.Select(r => r.GetSyntax()).OfType<ConstructorDeclarationSyntax>().FirstOrDefault();
 
         public override Microsoft.CodeAnalysis.Location? FullLocation => ReportingLocation;

csharp/extractor/Semmle.Extraction.CSharp/Entities/Conversion.cs

@@ -13,7 +13,7 @@ private Conversion(Context cx, IMethodSymbol init)
         public static new Conversion Create(Context cx, IMethodSymbol symbol) =>
             ConversionFactory.Instance.CreateEntityFromSymbol(cx, symbol);
 
-        public override Microsoft.CodeAnalysis.Location ReportingLocation
+        public override Microsoft.CodeAnalysis.Location? ReportingLocation
         {
             get
             {

csharp/extractor/Semmle.Extraction.CSharp/Entities/Method.cs

@@ -83,7 +83,7 @@ private void PopulateMethodBody(TextWriter trapFile)
                        if (block != null)
                            Statements.Block.Create(Context, block, this, 0);
                        else
-                           Expression.Create(Context, expr, this, 0);
+                           Expression.Create(Context, expr!, this, 0);
 
                        NumberOfLines(trapFile, BodyDeclaringSymbol, this);
                    });

csharp/extractor/Semmle.Extraction.CSharp/Extractor/CompilerVersion.cs

@@ -75,7 +75,7 @@ public CompilerVersion(Options options)
                 }
 
                 var versionInfo = FileVersionInfo.GetVersionInfo(SpecifiedCompiler);
-                if (!knownCompilerNames.TryGetValue(versionInfo.OriginalFilename, out var vendor))
+                if (!knownCompilerNames.TryGetValue(versionInfo.OriginalFilename ?? string.Empty, out var vendor))
                 {
                     SkipExtractionBecause("the compiler name is not recognised");
                     return;