Support custom streams with file name info

Mono.Cecil/IHaveAFileName.cs

@@ -0,0 +1,9 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace Mono.Cecil {
+	public interface IHaveAFileName {
+		string GetFileName ();
+	}
+}

Mono.Cecil/ModuleDefinition.cs

@@ -1289,11 +1289,14 @@ public static bool HasImage (this ModuleDefinition self)
 
 		public static string GetFileName (this Stream self)
 		{
-			var file_stream = self as FileStream;
-			if (file_stream == null)
+			switch (self) {
+			case IHaveAFileName withFileName:
+				return withFileName.GetFileName ();
+			case FileStream fs:
+				return Path.GetFullPath (fs.Name);
+			default:
 				return string.Empty;
-
-			return Path.GetFullPath (file_stream.Name);
+			}
 		}
 
 		public static TargetRuntime ParseRuntime (this string self)

Test/Mono.Cecil.Tests/ModuleTests.cs

@@ -351,5 +351,20 @@ public void ExceptionInWriteDoesNotKeepLockOnFile ()
 			// Ensure you can still delete the file
 			File.Delete (path);
 		}
+
+		class StreamWithAName : MemoryStream, IHaveAFileName {
+			public string GetFileName ()
+			{
+				return "Yes I have!";
+			}
+		}
+
+		[Test]
+		public void StreamImplementingIHaveAFileNameShouldReturnItAsIs ()
+		{
+			using (Stream stream = new StreamWithAName ()) {
+				Assert.AreEqual ("Yes I have!", stream.GetFileName ());
+			}
+		}
 	}
 }