Func<> getting the parameter info

c# lambda func
Vincent Dagpin picture Vincent Dagpin · Jul 17, 2013 · Viewed 7.9k times · Source

How to get the value of the passed parameter of the Func<> Lambda in C#

IEnumerable<AccountSummary> _data = await accountRepo.GetAsync();
string _query = "1011";
Accounts = _data.Filter(p => p.AccountNumber == _query);

and this is my extension method

public static ObservableCollection<T> Filter<T>(this IEnumerable<T> collection, Func<T, bool> predicate)
{
        string _target = predicate.Target.ToString();
        // i want to get the value of query here.. , i expect "1011"

        throw new NotImplementedException();
}

I want to get the value of query inside the Filter extension method assigned to _target

Answer

Fried picture Fried · Jul 17, 2013

If you want to get the parameter you will have to pass expression. By passing a "Func" you will pass the compiled lambda, so you cannot access the expression tree any more.

public static class FilterStatic
{
    // passing expression, accessing value
    public static IEnumerable<T> Filter<T>(this IEnumerable<T> collection, Expression<Func<T, bool>> predicate)
    {
        var binExpr = predicate.Body as BinaryExpression;
        var value = binExpr.Right;

        var func = predicate.Compile();
        return collection.Where(func);
    }

    // passing Func
    public static IEnumerable<T> Filter2<T>(this IEnumerable<T> collection, Func<T, bool> predicate)
    {
        return collection.Where(predicate);
    }
}

Testmethod

var accountList = new List<Account>
{
    new Account { Name = "Acc1" },
    new Account { Name = "Acc2" },
    new Account { Name = "Acc3" },
};
var result = accountList.Filter(p => p.Name == "Acc2");  // passing expression
var result2 = accountList.Filter2(p => p.Name == "Acc2");  // passing function

Related questions

converting a .net Func<T> to a .net Expression<Func<T>>

Going from a lambda to an Expression is easy using a method call... public void GimmeExpression(Expression<Func<T>> expression) { ((MemberExpression)expression.Body).Member.Name; // "DoStuff" } public void SomewhereElse() { GimmeExpression(() => thing.DoStuff()); } But I would …

Read More
Difference between Func<> with delegate and lambda expression

while deepening myself to more advanced features of C#, I came across some code, which I didn't exactly know the difference of. It's about these two lines: Func<string, int> giveLength = (text => text.Length); and Func<…

Read More
Assigning a Func to an Expression and vice versa

I was tampering with Expressions and I got confused at some points We can assign same LamdaExpression to both Expression and/or Func. But we cannot assign a Func to an Expression (or an Expression to Func). Why cannot we …

Read More