Add Message property to ExperimentalAttribute

src/libraries/System.Private.CoreLib/src/System/Diagnostics/CodeAnalysis/ExperimentalAttribute.cs

@@ -49,6 +49,15 @@ public ExperimentalAttribute(string diagnosticId)
         /// </remarks>
         public string DiagnosticId { get; }
 
+        /// <summary>
+        ///  Gets or sets an optional message associated with the experimental attribute.
+        /// </summary>
+        /// <value>The message that provides additional information about the experimental feature.</value>
+        /// <remarks>
+        ///  This message can be used to provide more context or guidance about the experimental feature.
+        /// </remarks>
+        public string? Message { get; set; }
+
         /// <summary>
         ///  Gets or sets the URL for corresponding documentation.
         ///  The API accepts a format string instead of an actual URL, creating a generic URL that includes the diagnostic ID.

src/libraries/System.Runtime/ref/System.Runtime.cs

@@ -8869,6 +8869,7 @@ public sealed partial class ExperimentalAttribute : System.Attribute
     {
         public ExperimentalAttribute(string diagnosticId) { }
         public string DiagnosticId { get { throw null; } }
+        public string? Message { get { throw null; } set { } }
         public string? UrlFormat { get { throw null; } set { } }
     }
     [System.AttributeUsageAttribute(System.AttributeTargets.Property, Inherited = false, AllowMultiple = true)]

src/libraries/System.Runtime/tests/System.Runtime.Tests/System/Diagnostics/CodeAnalysis/ExperimentalAttributeTests.cs

@@ -36,5 +36,20 @@ public void TestSetUrlFormat(string urlFormat)
             Assert.Equal("diagnosticId", attr.DiagnosticId);
             Assert.Equal(urlFormat, attr.UrlFormat);
         }
+
+        [Theory]
+        [InlineData(null)]
+        [InlineData("")]
+        [InlineData("This is an experimental feature")]
+        public void TestSetMessage(string message)
+        {
+            var attribute = new ExperimentalAttribute("diagnosticId")
+            {
+                Message = message
+            };
+
+            Assert.Equal("diagnosticId", attribute.DiagnosticId);
+            Assert.Equal(message, attribute.Message);
+        }
     }
 }