How to write LINQ operation where by myself?-CodePudding

I'm writing LINQ operations by myself. At the moment, I'm having trouble with where. Take a look at my code, this is MyWhere method:

public static T MyWhere<T>(this IEnumerable<T> myLst, Predicate<T> predicate)
{
    T whereItem = default;
    foreach (var item in myLst)
    {
        if (predicate.Invoke(item))
        {
            whereItem = item;
            continue;
        }
    }
    return whereItem;
}

This is in main method:

var myWhereItem = fruits.MyWhere(fruit => fruit.Length < 6);

        Console.Write("MyWhere: ");
        foreach (string item in myWhereItem)
        {
            Console.Write(myWhereItem   ", ");
        }

How do I write Where method, currently it's LastOrDefault?

CodePudding user response：

Your MyWhere method returns a single value, not an enumeration of values. In your example code you call MyWhere which returns a single string value, which you then try to assign to an explicitly-typed IEnumerable<string>.

If you check the LINQ definition for From you'll find that it actually returns an IEnumerable<TSource>. If you're trying to replicate the LINQ functionality (which is probably a Really Bad Idea) the simplest option (in modern C#) is to use an enumerable method:

public static IEnumerable<T> MyWhere<T>(this IEnumerable<T> sequence, Predicate<T> predicate)
{
    foreach (var item in sequence)
    {
        if (predicate(item))
            yield return item;
    }
}

(This is simply syntactic sugar (compiler-supported shorthand) to generate a complete IEnumerable<T> implementation in the background, making it easier to implement.)

If you're just trying to figure out how LINQ works, it's not hard to read the actual source code for it.