SingleOrDefault：在Custome结构上不返回null

Question

I have structure,

public struct Test
{
    public int int1;
    public string str;
}

and in my code I have,

List<Test> list = new List<Test>()
{ 
    new Test(){ int1 =1, str="abc" }, 
    new Test(){ int1 =2, str="abc" }
};

When I am trying to use SingleOrDefault on List<Test> list with search criteria int1 value equals 3

Test result = list.SingleOrDefault(o => o.int1 == 3);

Here result have value with default values, means int1 = 0 and str = null. Here I want null value if search criteria not satisfied. Anyone point me How I can do this?

Answer 1

You will not get null returned because Test is a struct, a value type. Change Test to a class and it will return null.

Answer 2

Value types are not nullable, so you either have to use a class, or a nullable Test? .

But if you want to stick with a struct , you should create a static field called Empty to check for empty values:

public struct Test
{
    public static readonly Test Emtpy = new Test();
    public int int1;
    public string str;

    public static bool operator ==(Test a, Test b)
    {
        return a.int1 == b.int1 && Equals(a.str, b.str);
    }

    public static bool operator !=(Test a, Test b)
    {
        return !(a==b);
    }
}

It's a convention that you will find across the .Net framework. If you later want to check for null (what you probably do), check for Test.Empty instead.

List<Test> list = new List<Test>(){ new Test(){ int1 =1,str="abc"}, new Test(){ int1 =2,str="abc"}};
Test result = list.SingleOrDefault(o => o.int1 == 3);

if (result != Test.Emtpy)
    ...

Answer 3

A dirty fix:

    Test result = list.FirstOrDefault(o => o.int1 == 3);

    if (result.Equals(default(Test)))
    {
        // not found
    }
    else
    {
        // normal work
    }

Use this only if you pretty sure that your original list never contains the default struct ( new Test() { int1 = 0, str = null } )

Answer 4

Test? result = list.Select(o => (?Test)o).SingleOrDefault(o => o.Value.int1 == 3);

It is not pretty, but it does its job. You might want to extract that pattern into a helper method.