c# Expression tree: call method with out-parameter?

Question

For example: I want to call the

Int32.TryParse(String numberStr, out Int32 result)

in Expression tree, but I do not know how to get the result of parsing.

Answer 1

You don't need to do anything special to call methods which take out parameters when using Expressions: just treat it as any other parameter, and the runtime takes care of it.

Here's an example of doing something like:

void Lambda(string input)
{
    int parsed;
    int.TryParse(input, out parsed);
    Console.WriteLine("Parsed: {0}", (object)parsed);
}

using expressions:

public static void Main()
{
    var inputParam = Expression.Parameter(typeof(string), "input");
    var parsedVar = Expression.Variable(typeof(int), "parsed");

    var tryParseCall = Expression.Call(
        typeof(int),
        "TryParse",
        null,
        inputParam,
        parsedVar); // <-- Here we pass 'parsedVar' as the 'out' parameter

    var writeLineCall = Expression.Call(
        typeof(Console),
        "WriteLine",
        null,
        Expression.Constant("Parsed: {0}"),
        Expression.Convert(parsedVar, typeof(object)));

    var lambda = Expression.Lambda<Action<string>>(
        Expression.Block(
            new[] { parsedVar },
            tryParseCall,
            writeLineCall),
        inputParam);

    var compiled = lambda.Compile();
    compiled("3");
}

See it working on dotnetfiddle