Question

I have a request to one of my actions in a controller that expects an enum as a key in the Querystring. I can see that the same is set in the QueryString but an exception is thrown by the server saying the same is not set and the error:

The parameters dictionary contains a null entry for parameter ‘enumVar’ of non-nullable type.

is thrown.

Here are the exception details:

Exception type: System.ArgumentException

Exception message: The parameters dictionary contains a null entry for parameter ‘enumVar’ of non-nullable type ‘NameSpace.Enums.MyEnum’ for method ‘System.Web.Mvc.ActionResult GetContent(Int64, NameSpace.Enums.MyEnum, System.String, Int32, Int32, Int32, Int32, Int64)’ in ‘NameSpace.Controllers.MyController’. An optional parameter must be a reference type, a nullable type, or be declared as an optional parameter.

Parameter name: parameters

Request URL: http://myawesomeurl.local/app/My/GetContent?abc=a_123&xyz=434&enumVar=EnumValue&teamId=a_123&a=0&b=1&c=3&qwerty=123&token=234234231

Request path: app/My/GetContent

Let me know if you need more information. I am not sure how I should address the issue.

Update:
My Enum defination:

public enum DisplayMode
{
    EnumValue,
    EnumValue1,
    EnumValue2
}

Also I should mention that the request fails only sometimes and not always and this is not consistent. This is an error that was logged in our exception db.

Answer 1

It looks like the Default Model Binder is not dailed into enumerations. Your best bet would be to subclass the DefaultModelBinder class and handle the scenario where a property is an enum.

public class U413ModelBinder : DefaultModelBinder 
{ 
    /// <summary> 
    /// Fix for the default model binder's failure to decode enum types when binding to JSON. 
    /// </summary> 
    protected override object GetPropertyValue(ControllerContext controllerContext, ModelBindingContext bindingContext, 
        PropertyDescriptor propertyDescriptor, IModelBinder propertyBinder) 
    { 
        var propertyType = propertyDescriptor.PropertyType; 
        if (propertyType.IsEnum) 
        { 
            var providerValue = bindingContext.ValueProvider.GetValue(bindingContext.ModelName); 
            if (null != providerValue) 
            { 
                var value = providerValue.RawValue; 
                if (null != value) 
                { 
                    var valueType = value.GetType(); 
                    if (!valueType.IsEnum) 
                    { 
                        return Enum.ToObject(propertyType, value); 
                    } 
                } 
            } 
        } 
        return base.GetPropertyValue(controllerContext, bindingContext, propertyDescriptor, propertyBinder); 
    } 
} 

Code and logic courtesy of this question.