diff --git a/csharp/src/Google.Protobuf.Test/RefStructCompatibilityTest.cs b/csharp/src/Google.Protobuf.Test/RefStructCompatibilityTest.cs
new file mode 100644
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+++ b/csharp/src/Google.Protobuf.Test/RefStructCompatibilityTest.cs
@@ -0,0 +1,119 @@
+#region Copyright notice and license
+// Protocol Buffers - Google's data interchange format
+// Copyright 2008 Google Inc.  All rights reserved.
+// https://developers.google.com/protocol-buffers/
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+#endregion
+
+using NUnit.Framework;
+using System.Diagnostics;
+using System;
+using System.Reflection;
+using System.IO;
+
+namespace Google.Protobuf
+{
+    public class RefStructCompatibilityTest
+    {
+        /// <summary>
+        /// Checks that the generated code can be compiler with an old C# compiler.
+        /// The reason why this test is needed is that even though dotnet SDK projects allow to set LangVersion,
+        /// this setting doesn't accurately simulate compilation with an actual old pre-roslyn C# compiler.
+        /// For instance, the "ref struct" types are only supported by C# 7.2 and higher, but even if
+        /// LangVersion is set low, the roslyn compiler still understands the concept of ref struct
+        /// and silently accepts them. Therefore we try to build the generated code with an actual old C# compiler
+        /// to be able to catch these sort of compatibility problems.
+        /// </summary>
+        [Test]
+        public void GeneratedCodeCompilesWithOldCsharpCompiler()
+        {
+            if (Environment.OSVersion.Platform != PlatformID.Win32NT)
+            {
+                // This tests needs old C# compiler which is only available on Windows. Skipping it on all other platforms.
+                return;
+            }
+
+            var currentAssemblyDir = Path.GetDirectoryName(typeof(RefStructCompatibilityTest).GetTypeInfo().Assembly.Location);
+            var testProtosProjectDir = Path.GetFullPath(Path.Combine(currentAssemblyDir, "..", "..", "..", "..", "Google.Protobuf.Test.TestProtos"));
+            var testProtosOutputDir = (currentAssemblyDir.Contains("bin/Debug/") || currentAssemblyDir.Contains("bin\\Debug\\")) ? "bin\\Debug\\net45" : "bin\\Release\\net45";
+
+            // If "ref struct" types are used in the generated code, compilation with an old compiler will fail with the following error:
+            // "XYZ is obsolete: 'Types with embedded references are not supported in this version of your compiler.'"
+            // We build the code with GOOGLE_PROTOBUF_REFSTRUCT_COMPATIBILITY_MODE to avoid the use of ref struct in the generated code.
+            var compatibilityFlag = "-define:GOOGLE_PROTOBUF_REFSTRUCT_COMPATIBILITY_MODE";
+            var sources = "*.cs";  // the generated sources from the TestProtos project
+            var args = $"-langversion:3 -target:library {compatibilityFlag} -reference:{testProtosOutputDir}\\Google.Protobuf.dll -out:{testProtosOutputDir}\\TestProtos.RefStructCompatibilityTest.OldCompiler.dll {sources}";
+            RunOldCsharpCompilerAndCheckSuccess(args, testProtosProjectDir);
+        }
+
+        /// <summary>
+        /// Invoke an old C# compiler in a subprocess and check it finished successful.
+        /// </summary>
+        /// <param name="args"></param>
+        /// <param name="workingDirectory"></param>
+        private void RunOldCsharpCompilerAndCheckSuccess(string args, string workingDirectory)
+        {  
+            using (var process = new Process())
+            {
+                // Get the path to the old C# 5 compiler from .NET framework. This approach is not 100% reliable, but works on most machines.
+                // Alternative way of getting the framework path is System.Runtime.InteropServices.RuntimeEnvironment.GetRuntimeDirectory()
+                // but it only works with the net45 target.
+                var oldCsharpCompilerPath = Path.Combine(Environment.GetEnvironmentVariable("WINDIR"), "Microsoft.NET", "Framework", "v4.0.30319", "csc.exe");
+                process.StartInfo.FileName = oldCsharpCompilerPath;
+                process.StartInfo.RedirectStandardOutput = true;
+                process.StartInfo.RedirectStandardError = true;
+                process.StartInfo.UseShellExecute = false;
+                process.StartInfo.Arguments = args;
+                process.StartInfo.WorkingDirectory = workingDirectory;
+
+                process.OutputDataReceived += (sender, e) =>
+                {
+                    if (e.Data != null)
+                    {
+                        Console.WriteLine(e.Data);
+                    }
+                };
+                process.ErrorDataReceived += (sender, e) =>
+                {
+                    if (e.Data != null)
+                    {
+                        Console.WriteLine(e.Data);
+                    }
+                };
+
+                process.Start();
+
+                process.BeginErrorReadLine();
+                process.BeginOutputReadLine();
+
+                process.WaitForExit();
+                Assert.AreEqual(0, process.ExitCode);
+            }
+        }
+    }
+}
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