Fix crash when Data Source starts with a comma

src/Microsoft.Data.SqlClient/src/Microsoft/Data/Common/AdapterUtil.cs

@@ -792,8 +792,19 @@ private static bool IsEndpoint(string dataSource, string prefix)
                 length = foundIndex;
             }
 
-            // check for the instance name
-            foundIndex = dataSource.LastIndexOf('\\', length - 1, length - 1);
+            // Safeguard LastIndexOf call to avoid ArgumentOutOfRangeException when length is 0
+            if (length > 0)
+            {
+                // check for the instance name
+                foundIndex = dataSource.LastIndexOf('\\', length - 1, length - 1);
+            }
+            else
+            {
+                foundIndex = -1;
+            }
+
+
+
             if (foundIndex > 0)
             {
                 length = foundIndex;

src/Microsoft.Data.SqlClient/tests/FunctionalTests/SqlConnectionTest.cs

@@ -1082,6 +1082,20 @@ public void ConnectionRetryForNonAzureEndpoints()
             Assert.Equal(1, (int)field.GetValue(cn));
         }
 
+
+
+        [Fact]
+        public void ConnectionString_WithOnlyComma()
+        {
+            // Test Case for https://github.com/dotnet/SqlClient/issues/3110
+            // Validates that a single-comma Data Source (e.g., "Data Source=,") no longer causes ArgumentOutOfRangeException
+            // Instead, it should throw a SqlException indicating a connection failure
+
+            SqlConnection cn = new SqlConnection("Data Source=,;Initial Catalog=master;Integrated Security=True");
+            Assert.Throws<SqlException>(() => { cn.Open(); });
+
+        }
+
         [Theory]
         [InlineData("myserver.database.windows.net")]
         [InlineData("myserver.database.cloudapi.de")]