C#: Fix a few alerts

Author: hvitved

csharp/extractor/Semmle.Extraction.Tests/Layout.cs

@@ -185,7 +185,7 @@ public static void Emit(this TrapWriter trapFile, string s)
 
         class StringTrapEmitter : ITrapEmitter
         {
-            string Content;
+            readonly string Content;
             public StringTrapEmitter(string content)
             {
                 Content = content;

csharp/extractor/Semmle.Extraction.Tests/Options.cs

@@ -191,7 +191,7 @@ public void Fast()
         [Fact]
         public void ArchiveArguments()
         {
-            var sw = new StringWriter();
+            using var sw = new StringWriter();
             var file = Path.GetTempFileName();
 
             try

csharp/extractor/Semmle.Extraction/Entities/GeneratedLocation.cs

@@ -32,7 +32,7 @@ public override void WriteId(TextWriter trapFile)
 
         class GeneratedLocationFactory : ICachedEntityFactory<string?, GeneratedLocation>
         {
-            public static GeneratedLocationFactory Instance = new GeneratedLocationFactory();
+            public static readonly GeneratedLocationFactory Instance = new GeneratedLocationFactory();
 
             public GeneratedLocation Create(Context cx, string? init) => new GeneratedLocation(cx);
         }

csharp/extractor/Semmle.Util.Tests/CanonicalPathCache.cs

@@ -21,15 +21,7 @@ public CanonicalPathCacheTest()
             // Change directories to a directory that is in canonical form.
             Directory.SetCurrentDirectory(cache.GetCanonicalPath(Path.GetTempPath()));
 
-            if (Win32.IsWindows())
-            {
-                root = @"X:\";
-            }
-            else
-            {
-                root = "/";
-            }
-
+            root = Win32.IsWindows() ? @"X:\" : "/";
         }
 
         void IDisposable.Dispose()
@@ -143,10 +135,10 @@ public void CanonicalPathCacheSize()
             Assert.Equal(0, cache.CacheSize);
 
             // The file "ABC" will fill the cache with parent directory info.
-            string cp = cache.GetCanonicalPath("ABC");
+            cache.GetCanonicalPath("ABC");
             Assert.True(cache.CacheSize == 2);
 
-            cp = cache.GetCanonicalPath("def");
+            string cp = cache.GetCanonicalPath("def");
             Assert.Equal(2, cache.CacheSize);
             Assert.Equal(Path.GetFullPath("def"), cp);
         }

csharp/extractor/Semmle.Util.Tests/LongPaths.cs

@@ -93,7 +93,7 @@ public void Replace()
             Assert.Equal("def", File.ReadAllText(shortPath));
         }
 
-        byte[] buffer1 = new byte[10] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
+        readonly byte[] buffer1 = new byte[10] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
 
         [Fact]
         public void CreateShortStream()