Question

I am using ServiceStack to create a service which accepts request from and HTML form (POSTed). One of the DTO properties is an Enum, and when the input doesn’t match the Enum members, I get the following exception:

Error occured while Processing Request: KeyValueDataContractDeserializer: Error converting to type: Requested value 'MyValue' was not found.
System.Runtime.Serialization.SerializationException: KeyValueDataContractDeserializer: Error converting to type: Requested value 'MyValue' was not found. ---> System.ArgumentException: Requested value 'MyValue' was not found.
   at System.Enum.TryParseEnum(Type enumType, String value, Boolean ignoreCase, EnumResult& parseResult)
   at System.Enum.Parse(Type enumType, String value, Boolean ignoreCase)
   at ServiceStack.ServiceModel.Serialization.StringMapTypeDeserializer.PopulateFromMap(Object instance, IDictionary`2 keyValuePairs) 

How can I intercept this exception and handle it myself in my service code?

Answer 1

There are a couple of ways to handle this situation:

You can make the DTO Enum property a string (since everything can successfully deserialize into a string 🙂 and then convert that yourself manually i.e.

using ServiceStack.Common; //ToEnum<> is an extension method

public class RequestDto
{
    public string EnumString { get; set; } 
}

public override object OnGet(RequestDto request)
{
    MyEnum defaultValue = MyEnum.None;
    try {
        defaultValue = request.EnumString.ToEnum<MyEnum>();
    } catch {}
}

The other alternative is to completely remove it from the request DTO and get value manually from the IHttpRequest Context like:

public class RequestDto {}
public override object OnGet(RequestDto request)
{
    MyEnum enumValue = MyEnum.DefaultValue;
    try {
        var enumStr = base.RequestContext.Get<IHttpRequest>().QueryString["EnumString"];
        enumValue = enumStr.ToEnum<MyEnum>();
    } catch {}
}

I generally discourage the use of enums on DTOs for many reasons, the primary one being on XML/SOAP endpoints the XSD treats them as a restricted set of values which is a pain in when trying iteratively to evolve your web services as you will need to re-gen the clients to add a new value.

By convention the way I deal with it is to have all enums as strings but provide some metadata on the DTO which points to the target type (which helps in VS.NET/R# navigation and metadata tools).

public class RequestDto
{
    [References(typeof(MyEnum))]
    public string EnumString { get; set; } 
}