c# 在方法调用后告诉 Visual Studio 属性不是 null

Question

How can I tell Visual Studio IntelliSense that property of property is not null after calling its method

Inner class:

class Inner
{
    int? Property { get; set; }

    [MemberNotNull(nameof(Property))]
    void Initialize
    {
        Property = 42;
    }
}

Caller class:

class Caller
{
    // We know here that constructor is always called before other methods
    public Caller()
    {
        InnerObj = new Inner();

        // We know here that InnerObj.Property is always not null
        InnerObj.Initialize();
    }

    public Inner InnerObj { get; }

    public void MethodWithNullWarning()
    {
        // Null Warning
        InnerObj.Property.ToString();
    }
}

Answer 1

You can use the null-forgiving operator .

InnerObj!.Property.ToString();

Quoted from docs:

Available in C# 8.0 and later, the unary postfix, operator is the null-forgiving, or null-suppression. operator, In an enabled nullable annotation context: you use the null-forgiving operator to declare that expression x of a reference type isn't null. x.. The unary prefix ! operator is the logical negation operator.

Answer 2

Assigning the property in the constructor doesn't guarantee it will not be null.

Let's add the following method:

public void MakeItNull() { InnerObj.Property = null; }

Then:

Caller caller = new ();

caller.MakeItNull();
caller.MethodWithNullWarning();

Boom!

---

Solutions

Here are some options.

1. !

! (null-forgiving) operator (C# reference)

InnerObj.Property!.ToString();

Please note that this doesn't protect from a null reference exception if the value is actually null.

2. Have a constructor

class Inner
{
    public string Property { get; set; } // string, not string!

    public Inner() {
        Property = "42";
    }
}

3. Use a different property for external access.

class Inner
{
    private string? RealProperty { get; set; }


    public string Property => RealProperty!;

    public void Initialize() {
        RealProperty = "42";
    }
}