Prevent duplicate dispose exception (#349)

* Prevent duplicate dispose exception

* Address PR comments

Author: gitting

System.IO.Abstractions.TestingHelpers.Tests/MockFileStreamTests.cs

@@ -74,5 +74,35 @@ public void MockFileStream_Constructor_ReadTypeNotWritable()
             Assert.IsFalse(stream.CanWrite);
             Assert.Throws<NotSupportedException>(() => stream.WriteByte(1));
         }
+
+        [Test]
+        public void MockFileStream_Close_MultipleCallsDontThrow()
+        {
+            var fileSystem = new MockFileSystem();
+            var path = XFS.Path("C:\\test");
+            fileSystem.AddFile(path, new MockFileData("Bla"));
+            var stream = fileSystem.File.OpenRead(path);
+
+            // Act
+            stream.Close();
+
+            // Assert
+            Assert.DoesNotThrow(() => stream.Close());
+        }
+
+        [Test]
+        public void MockFileStream_Dispose_MultipleCallsDontThrow()
+        {
+            var fileSystem = new MockFileSystem();
+            var path = XFS.Path("C:\\test");
+            fileSystem.AddFile(path, new MockFileData("Bla"));
+            var stream = fileSystem.File.OpenRead(path);
+
+            // Act
+            stream.Dispose();
+
+            // Assert
+            Assert.DoesNotThrow(() => stream.Dispose());
+        }
     }
 }

System.IO.Abstractions.TestingHelpers/MockFileStream.cs

@@ -6,6 +6,7 @@ public class MockFileStream : MemoryStream
         private readonly IMockFileDataAccessor mockFileDataAccessor;
         private readonly string path;
         private readonly bool canWrite = true;
+        private bool disposed;
 
         public enum StreamType
         {
@@ -53,8 +54,13 @@ public override void Close()
 #else
         protected override void Dispose(bool disposing)
         {
+            if (disposed)
+            {
+                return;
+            }
             InternalFlush();
             base.Dispose(disposing);
+            disposed = true;
         }
 #endif