add mono csharp

runtimes/default.nix

@@ -33,4 +33,5 @@ args: {
     "nim" = import ./nim.nix args;
     "nasm" = import ./nasm.nix args;
     "nasm-nasm64" = import ./nasm-nasm64.nix args;
+    "mono-csharp" = import ./mono-csharp.nix args;
 }

runtimes/mono-csharp.nix

@@ -0,0 +1,49 @@
+{pkgs, piston, ...}:
+let
+    pkg = pkgs.mono; 
+in piston.mkRuntime {
+    language = "csharp";
+    version = pkg.version;
+    runtime = "mono";
+
+    aliases = [
+        "mono"
+        "mono-csharp"
+        "mono-c#"
+        "mono-cs"
+        "c#"
+        "cs"
+    ];
+
+    compile = ''
+        rename 's/$/\.cs/' "$@" # Add .cs extension
+        ${pkg}/bin/csc -out:out *.cs
+    '';
+
+    run = ''
+        shift
+        ${pkg}/bin/mono out "$@"
+    '';
+
+    tests = [
+        (piston.mkTest {
+            files = {
+                "test.cs" = ''
+                    using System;
+
+                    public class Test
+                    {
+                        public static void Main(string[] args)
+                        {
+                            Console.WriteLine("OK");
+                        }
+                    }
+                '';
+            };
+            args = [];
+            stdin = "";
+            packages = [];
+            main = "test.cs";
+        })
+    ];
+}