Add Csharp
diff --git a/tools/interop_matrix/client_matrix.py b/tools/interop_matrix/client_matrix.py
index 18e9341..131c4a7 100644
--- a/tools/interop_matrix/client_matrix.py
+++ b/tools/interop_matrix/client_matrix.py
@@ -31,6 +31,7 @@
     'node': ['node'],
     'ruby': ['ruby'],
     'php': ['php', 'php7'],
+    'csharp': ['csharp', 'csharpcoreclr'],
 }
 
 # Dictionary of releases per language.  For each language, we need to provide
@@ -95,4 +96,13 @@
         'v1.4.2',
         'v1.6.6',
     ],
+   'csharp': [
+        # Fail to build docker image with v1.3.9 and earlier.
+        #'v1.0.1',
+        #'v1.1.4',
+        #'v1.2.5',
+        #'v1.3.9',
+        'v1.4.2',
+        'v1.6.6',
+    ],
 }