Cache compile from Expression<Func<T>>

Question

I have a class that I use for the checking method arguments, which you call in the form:

public void SomeMethod(string anArg)
{
    Ensure.ArgumentNotNull(() => anArg);
}

If the argument is null then an ArgumentNullException with the name of the property is thrown. This is done like so:

public static void ArgumentNotNull<T>(Expression<Func<T>> expression) where T : class 
{
    var value = expression.Compile()();
    if (value == null)
    {
        throw new ArgumentNullException(expression.GetMemberName());
    }
}

Where GetMemberName is an extension method I've written.

The problem I'm having is that the call to Compile is very slow, so I'd like to cache the result, but I don't seem to be able to come up with a cache key that will be unique enough to prevent cache conflicts, but not so unique that the cache becomes invalid.

My best effort so far is:

internal static class ExpressionCache<T>
{
    private static readonly Dictionary<string, Func<T>> Cache = new Dictionary<string, Func<T>>();

    public static Func<T> CachedCompile(Expression<Func<T>> targetSelector)
    {
        Func<T> cachedFunc;
        var cacheKey = targetSelector + targetSelector.Body.ToString();

        if (!Cache.TryGetValue(cacheKey, out cachedFunc))
        {
            cachedFunc = targetSelector.Compile();
            Cache[cacheKey] = cachedFunc;
        }

        return cachedFunc;
    }
}

But this still causes cache key conflicts. What might be a better approach?

Answer 1

Where do the exrpessions come from, are they created new? If they are reused, you could just use the expression itself as the key.:

internal static class ExpressionCache<T>
{
    private static readonly Dictionary<Expression<Func<T>, Func<T>> Cache = new Dictionary<Expression<Func<T>, Func<T>>();

    public static Func<T> CachedCompile(Expression<Func<T>> targetSelector)
    {
        Func<T> cachedFunc;
        if (!Cache.TryGetValue(targetSelector, out cachedFunc))
        {
            cachedFunc = targetSelector.Compile();
            Cache[targetSelector] = cachedFunc;
        }

        return cachedFunc;
    }
}

Else you could snoop around int he source code for the DLR http://dlr.codeplex.com/, I believe they address this kind of questions quite well.