Dictionary Tkey Tvalue Trygetvalue And Anonymous Types

One of the methods I find to be the most useful in .Net is the method Dictionary<TKey,TValue>.TryGetValue. This method is a nice compromise between performance, explicit return vs. exception, and a being verbal about the chance of failure. It returns false on failure and uses an out parameter to return the actual requested value. This leads to the following elegant pattern

Student value;
if (map.TryGetValue("SomeKey", out value)) {
    // Value is present
}
else {
    // Value is not present
}

This works great right up until have a Dictionary of anonymous types. The TryGetValue pattern functions on out parameters which do not work well with type inference in C# and hence anonymous types. Type inference requires that the value be declared with a corresponding initialization expression. But any out call forces the declaration of the type and the initialization expression to be different statements breaking any chance of type inference.

For example take the following code which builds up a Dictionary object where the value is typed to be an anonymous type ¹

var query = from it in GetStudents()
            where it.LastName.StartsWith(lastNamePrefix)
            select new { FirsName = it.FirstName, LastName = it.LastName };
// ...
var map = query.ToDictionary(x => x.FirsName);

// How to use TryGetValue?  
WhatDoIPutHere' value;
if ( map.TryGetValue("Joe", out value) {
    // ...
}

To fix this problem we need to write a wrapper around TryGetValue which allows us to combine both the presence or absence of the entry in the Dictionary and the resulting looked up value if present.’? Within our wrapper method we can use type inference tricks to avoid naming the anonymous type directly. To combine the values could construct a new type say TryGetValueResult<TValue>

struct TryGetValueResult<TValue> {
    public readonly bool Success;
    public readonly TValue Value;
    public TryGetValueResult(bool success, TValue value) {
        Success = success;
        Value = value;
    }
}

But I find this to be a bit heavy handed for a simple return. Instead I prefer to combine the data with the new Tuple<T1,T2> type introduced in 4.0.’? This type is designed to be a light weight method for combining two related values into a single instance. Perfect for this type of method.

public static Tuple<bool, TValue> TryGetValue<TKey, TValue>(
    this Dictionary<TKey, TValue> map, 
    TKey key) {

    TValue value;
    var ret = map.TryGetValue(key, out value);
    return Tuple.Create(ret, value);
}

Now that we’ve built our wrapper method we can go back to the original code sample and use it to access the anonymous type values

var tuple = map.TryGetValue("Joe");
if (tuple.Item1) {
    Console.WriteLine(tuple.Item2);
}

This pattern is not limited strictly to TryGetValue. It’s fairly applicable anytime you need to combine a return value and one or more out parameters into a single value for reasons of type inference.