为何List<T>.FindAll()仅接受Predicate<TSource>而非Func<TSource, bool>？

Great question—this is one of those .NET quirks that boils down to historical context and how delegate types work in C#. Let's break it down clearly:

The Core Issue: Delegate Type Compatibility #

Even though Func<TSource, bool> and Predicate<TSource> have identical signatures (both take a TSource parameter and return a bool), they are completely separate delegate types in .NET. C# doesn't allow implicit conversion between different delegate types—even if their method signatures match. That's exactly why you're seeing that frustrating error:

Argument 1: cannot convert from 'System.Func' to 'System.Predicate'

The compiler sees them as distinct types, so it won't automatically bridge the gap for you.

The Historical Backstory #

Here's why this mismatch exists in the first place:

• List<T> launched in .NET 2.0, alongside the Predicate<TSource> delegate. Back then, there was no generic Func<...> delegate family—those didn't arrive until .NET 3.5, when LINQ was added to the framework.
• Microsoft created Predicate<TSource> specifically for List's filtering methods (like FindAll, Exists, Find) because there was no general-purpose generic delegate for boolean-returning functions yet.
• By the time Func<TSource, bool> came around, List<T> was already widely used in existing code. Changing the parameter type to Func would have broken backward compatibility for thousands of projects, so Microsoft left the methods as-is.

How to Work Around It #

You have a couple of straightforward options:

1. Explicitly Convert Func to Predicate #

You can explicitly create a Predicate<TSource> instance from your lambda, or cast the lambda directly:

// Option A: Explicitly instantiate Predicate
List<Book> cheapBooks = books.FindAll(new Predicate<Book>(book => book.Price < 10));

// Option B: Cast the lambda directly
List<Book> cheapBooks = books.FindAll((Predicate<Book>)(book => book.Price < 10));

2. Use LINQ's Where Method (Recommended) #

If you're using .NET 3.5 or later, skip FindAll entirely and use LINQ's Where method—it accepts Func<TSource, bool> natively. Just remember to add using System.Linq; and call ToList() to get a List<T> result:

using System.Linq;

List<Book> cheapBooks = books.Where(book => book.Price < 10).ToList();

This is often the cleaner approach, especially if you're already using other LINQ methods in your code.

内容的提问来源于stack exchange，提问作者Nashmár