How to validate yield return default in IEnumerable?

问题

I am trying to check whether IEnumerable<> is null or empty but somehow my if check always fails whenever it is empty.

    private bool Update()
    {
        IEnumerable<RecordHolder> recordHolders = GetData();
        // below  check doesn't work
        if (recordHolders == null || !recordHolders.Any())
            return false;

        // .. some other code
    }

    public IEnumerable<RecordHolder> GetData()
    {
        var isSuccess = PullRemote(url);
        if (!isSuccess.Result) { yield return default; }

        // .. some other code
    }

Whenever my GetData() method returns yield return default then the subsequent if check in Update method doesn't work. Anything wrong I am doing?

I thought my if check in Update method will be able to catch yield return default but I am wrong looks like.

回答1:

I think you are looking for yield break instead of yield return default. Alternatively, you can do the first part of the operation in a regular method (not an iterator block), and special-case the return for failure cases (Array.Empty<T>() is a common choice), deferring to an iterator block (or a Select projection) for the success cases.

回答2:

When you do yield return default you are returning the default value of RecordHolder which is null.

So your IEnumerable will contain 1 item which is null.

Try returning Enumerable.Empty() or Array.Empty(); instead of your yield return default.

Example

using System;
using System.Collections.Generic;
using System.Linq;
                
public class Program
{
    public static void Main()
{
    var result = GetData();
    Console.WriteLine(result.Count()); // Prints 1
}

public static IEnumerable<MyClass> GetData()
{
    yield return default;
}

public class MyClass
{
        public string MyProperty {get;set;}
    }
}

Example here: https://dotnetfiddle.net/vVefPx

来源：https://stackoverflow.com/questions/63312797/how-to-validate-yield-return-default-in-ienumerable

标签

ASP.NET

ienumerable