c# 表达式树：使用输出参数调用方法？

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

Answer 2

The result is stored in the variable result (as you named in your out parameter). Try parse attempts to parse your input, and if it succeeds, it returns true and puts the resulting value in the named out parameter. Otherwise, it returns false and the out parameter is rendered invalid. You would use it like so.

if (Int32.TryParse(String numberStr, out Int32 result)) {
    // do something with `result`
}

// something went wrong (failure state - maybe with an `else`?)

Answer 3

Can you please try it with underscore "_"

List<string> lstNumbs = new List<string>() {"f", "d", "7", "4", "5", "4", "2"};

var result = lstNumbs.Where(c => Int32.TryParse(c, out int _)).Select(s => s);

foreach (var item in result)
{
    Console.WriteLine(item);
}

Demo: https://dotnetfiddle.net/GdbgIZ