Don't enumerate fields on generic type Def (#77)

* Don't enumerate fields on generic type Def

Prevent exception being thrown when generic type definition is not a constructed generic type when trying to retrieve field values.

Added TODO with suggestion as to how to diagnose type arguments for generic type definition to create a constructed generic type.

* add test demonstrating error

src/Ben.Demystifier/EnhancedStackTrace.Frames.cs

@@ -135,6 +135,12 @@ namespace System.Diagnostics
                 if (methodDisplayInfo.IsLambda && type != null)
                 {
                     if (methodName == ".cctor")
+                    {
+                        if (type.IsGenericTypeDefinition && !type.IsConstructedGenericType)
+                        {
+                            // TODO: diagnose type's generic type arguments from frame's "this" or something
+                        }
+                        else
                         {
                             var fields = type.GetFields(BindingFlags.Static | BindingFlags.Public | BindingFlags.NonPublic);
                             foreach (var field in fields)
@@ -155,6 +161,7 @@ namespace System.Diagnostics
                         }
                     }
                 }
+            }
 
             if (subMethodName != methodName)
             {

test/Ben.Demystifier.Test/GenericMethodDisplayStringTests.cs

@@ -0,0 +1,42 @@
+using System;
+using System.Diagnostics;
+using System.Threading.Tasks;
+using Xunit;
+
+namespace Ben.Demystifier.Test
+{
+    public class GenericMethodDisplayStringTests
+    {
+        private static class Example<T>
+        {
+            // ReSharper disable once StaticMemberInGenericType
+            public static readonly StackFrame StackFrame; 
+            
+            static Example()
+            {
+                var fun = new Func<StackFrame>(() => new StackFrame(0, true));
+                
+                StackFrame = fun();
+                
+            }
+
+        }
+
+        [Fact]
+        public void DiagnosesGenericMethodDisplayString()
+        {
+            var sf = Example<Type>.StackFrame;
+
+            try
+            {
+                var s = EnhancedStackTrace.GetMethodDisplayString(sf.GetMethod());
+                Assert.True(true, "Does not throw exception when diagnosing generic method display string.");
+            }
+            catch (Exception ioe)
+            {
+                Assert.True(false, "Must not throw an exception when diagnosing generic method display string.");
+            }
+
+        }
+    }
+}