Cannot convert from 'method group' to 'Func'

Question

I am implementing a very simple rules engine that enforces some specifications dynamically at runtime.

The actual rules are stored in Tuples, and I have trouble storing a delegate to the string.EndsWith function.

The following code works for testing string equality, and returns False as expected ("A" is different from "B"):

var rule = new Tuple<string, Func<string, string, bool>, string>("A", string.Equals, "B");
Console.WriteLine(rule.Item2.Invoke(rule.Item1, rule.Item3));

However, I cannot figure out how to adapt this code to use the string.EndsWith function instead of string.Equals.

The following code does not compile and issues a Cannot convert from 'method group' to 'Func<string, string, bool>' error message in Visual Studio.

var rule = new Tuple<string, Func<string, string, bool>, string>("A", string.EndsWith, "B");
Console.WriteLine(rule.Item2.Invoke(rule.Item1, rule.Item3));

I did search before asking this question, but I cannot understand the answers provided in How do I fix 'compiler error - cannot convert from method group to System.Delegate'? or Cannot convert from method group to System.Func<string>. I do not see how to apply these to my problem.

Just look at the signature of the method. None of them match the delegate, exactly as the error message is telling you. — Servy, Feb 10 '16 at 16:51
Also let start from `string.EndsWith` - there is no such method, i.e. the following does not compile `bool test = string.EndsWith("ABC", "C");`, right? — Ivan Stoev, Feb 10 '16 at 17:07
@Servy I thought the error message was telling me something different. But it makes sense now, you are right. — Laurent Vaylet, Feb 10 '16 at 17:25
@IvanStoev You are perfectly right. I wrongly assumed the methods were following the same pattern. — Laurent Vaylet, Feb 10 '16 at 17:26

score 7 · Accepted Answer · edited May 23 '17 at 12:33

String.Equals and String.EndsWith have different method signatures and must be called differently.

Specifically, String.Equals is static and takes two strings and returns a bool. String.EndsWith is an instance method taking one string and returns a bool

You can resolve your issue by wrapping the String.EndsWith call in a lambda to change the signature to take two strings and return a bool:

var rule = new Tuple<string, Func<string, string, bool>, string>
    ("AB", (string a, string b) => a.EndsWith(b), "B");
Console.WriteLine(rule.Item2.Invoke(rule.Item1, rule.Item3));

In general, the error means that there is no way the compiler can interpret string.EndsWith as a Func<string, string, bool>. You may find this answer about what is a method group helpful to understanding the error message.

Cannot convert from 'method group' to 'Func'

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