C#: Fix compiler warnings.

Author: michaelnebel

csharp/autobuilder/Semmle.Autobuild.CSharp.Tests/BuildScripts.cs

@@ -327,7 +327,7 @@ public void TestOr1()
             Assert.Equal("codeql", startCallbackIn[0]);
             Assert.Equal("", endCallbackIn[0]);
             Assert.Equal(0, endCallbackReturn[0]);
-            Assert.Equal(1, endCallbackReturn.Count);
+            Assert.Single(endCallbackReturn);
         }
 
         [Fact]

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

@@ -20,7 +20,10 @@ public UsingDirective(Context cx, UsingDirectiveSyntax usingDirective, Namespace
 
         protected override void Populate(TextWriter trapFile)
         {
-            var info = Context.GetModel(node).GetSymbolInfo(node.Name);
+            // This is guaranteed to be non-null as we only deal with "using namespace" not "using X = Y"
+            var name = node.Name!;
+
+            var info = Context.GetModel(node).GetSymbolInfo(name);
 
             if (node.StaticKeyword.IsKind(SyntaxKind.None))
             {
@@ -33,7 +36,7 @@ protected override void Populate(TextWriter trapFile)
                 }
                 else
                 {
-                    Context.Extractor.MissingNamespace(node.Name.ToFullString(), Context.FromSource);
+                    Context.Extractor.MissingNamespace(name.ToFullString(), Context.FromSource);
                     Context.ModelError(node, "Namespace not found");
                     return;
                 }

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

@@ -263,7 +263,7 @@ private static bool FileIsUpToDate(string src, string dest)
                 File.GetLastWriteTime(dest) >= File.GetLastWriteTime(src);
         }
 
-        private void AnalyseNamespace(Context cx, INamespaceSymbol ns)
+        private static void AnalyseNamespace(Context cx, INamespaceSymbol ns)
         {
             foreach (var memberNamespace in ns.GetNamespaceMembers())
             {