Throw descriptive exception if multiple attributes applied

Author: captainsafia

src/Mvc/Mvc.Core/src/ModelBinding/Metadata/ModelAttributes.cs

@@ -225,6 +225,14 @@ public static ModelAttributes GetAttributesForParameter(ParameterInfo parameterI
         // To avoid this, we call `GetCustomAttributes` directly
         // to avoid examining the inheritance hierarchy.
         // See https://source.dot.net/#System.Private.CoreLib/src/System/Attribute.CoreCLR.cs,677
-        return type.GetCustomAttributes<ModelMetadataTypeAttribute>(inherit: false).SingleOrDefault()?.MetadataType;
+        var modelMetadataTypeAttributes = type.GetCustomAttributes<ModelMetadataTypeAttribute>(inherit: false);
+        try
+        {
+            return modelMetadataTypeAttributes?.SingleOrDefault()?.MetadataType;
+        }
+        catch (InvalidOperationException e)
+        {
+            throw new InvalidOperationException("Only one ModelMetadataType attribute is permitted per type.", e);
+        }
     }
 }

src/Mvc/Mvc.Core/test/ModelBinding/Metadata/ModelAttributesTest.cs

@@ -287,6 +287,18 @@ public void GetAttributesForProperty_WithModelType_IncludesTypeAttributes()
             attribute => Assert.IsType<ClassValidator>(attribute));
     }
 
+    [Fact]
+    public void GetAttributeForProperty_WithModelType_HandlesMultipleAttributesOnType()
+    {
+        // Arrange
+        var modelType = typeof(InvalidBaseViewModel);
+        var property = modelType.GetRuntimeProperties().FirstOrDefault(p => p.Name == nameof(BaseModel.RouteValue));
+
+        // Assert
+        var exception = Assert.Throws<InvalidOperationException>(() => ModelAttributes.GetAttributesForProperty(modelType, property));
+        Assert.Equal("Only one ModelMetadataType attribute is permitted per type.", exception.Message);
+    }
+
     [ClassValidator]
     private class BaseModel
     {
@@ -335,6 +347,10 @@ private class BaseViewModel
         public string RouteValue { get; set; }
     }
 
+    [ModelMetadataType<BaseModel>]
+    [ModelMetadataType(typeof(BaseModel))]
+    private class InvalidBaseViewModel : BaseViewModel { }
+
     [ModelMetadataType<DerivedModel>]
     private class DerivedViewModel : BaseViewModel
     {