Better error message for Response<T> implicit cast from null (#25580)

sdk/core/Azure.Core/src/ResponseOfT.cs

@@ -1,6 +1,7 @@
 // Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT License.
 
+using System;
 using System.ComponentModel;
 using System.Diagnostics;
 
@@ -32,7 +33,17 @@ public abstract class Response<T>
         /// Returns the value of this <see cref="Response{T}"/> object.
         /// </summary>
         /// <param name="response">The <see cref="Response{T}"/> instance.</param>
-        public static implicit operator T(Response<T> response) => response.Value;
+        public static implicit operator T(Response<T> response)
+        {
+            if (response == null)
+            {
+#pragma warning disable CA1065 // Don't throw from cast operators
+                throw new ArgumentNullException(nameof(response), $"The implicit cast from Response<{typeof(T)}> to {typeof(T)} failed because the Response<{typeof(T)}> was null.");
+#pragma warning restore CA1065
+            }
+
+            return response.Value;
+        }
 
         /// <inheritdoc />
         [EditorBrowsable(EditorBrowsableState.Never)]

sdk/core/Azure.Core/tests/ResponseTests.cs

@@ -31,6 +31,17 @@ public void ValueObtainedFromCast()
             Assert.AreEqual("test_name", value.Name);
         }
 
+        [Test]
+        public void ImplicitCastFromResponseTToNullFails()
+        {
+            Response<string> response = null;
+            var exception = Assert.Throws<ArgumentNullException>(() =>
+            {
+                string s = response;
+            });
+            StringAssert.StartsWith("The implicit cast from Response<System.String> to System.String failed because the Response<System.String> was null.", exception.Message);
+        }
+
         [Test]
         public void ToStringsFormatsStatusAndValue()
         {