What is the result of initiating members with default! statement in C# .net?

Question

Yes, I tried to google around and found everything possible from null-coalescing operator to default keyword, which everything kind of bypasses (as of now) my curiosity about these statements:

public record Person
{
    public string FirstName { get; init; } = default!;
    public string LastName { get; init; } = default!;
};

The snippet comes from this MS official site .

So while I can try out what this code yelds in runtime record instantiation, can anyone please generally explain what is the beauty and magic behind the default! statements?

Thank you.

Answer 1

default keyword on it's own says use the default value for this type, in this case a null since string is a reference type.

The ! suffix is the null forgiving operator . It suppresses any warnings about a value potentially being null. Without it in this code, you would the following warning which is a feature of nullable reference types :

CS8625: Cannot convert null literal to non-nullable reference type

Answer 2

There are two things to consider

1. The default keyword denotes the value of a type where all the bits are set to 0 . Eg for an int this is 0 , for a double it is 0.0 and for a reference type like string , this is null .
2. In a Nullable context the two properties are non-nullable, therefore, you are in principle not allowed to assign them null . If you know it better than the compiler, you can use the ! (null-forgiving) operator to tell the compiler that null is okay in this case.

Why assign default! , ie, null to those properties? Declaring a property with an init accessor does not ensure that the constructor will be called with an object initializer setting these properties to a non-null value. Even if you know that your code will be doing it. The purpose of this = default! property initializer is simply to suppress any compiler warnings about this issue.

C# 11 will introduce a required modifier to force initialization of a property and to suppress this compiler warning.

public required string FirstName { get; init; }

A code using this property will not compile until you either initialize the property in the constructor or in an object initializer.

Answer 3

As pointed out by @SouXin, you can use this official documentation link to check the default value of each C# type.

However, in your case, since string is a reference type, default! is the same as null! . This assignment is used to tell the compiler that the developer is sure that the property is never null, by using ! , which suppresses the null warning. This is part of the nullable feature.

Otherwise, the compiler would return warning, saying that FistName and LastName can be null , and thus, should be of type string? .

This is just an intermediate solution, starting with C# 11 , the recommended solution of telling the compiler that a property won't be null is by using the required keyword.

So instead of having:

public string FirstName { get; init; } = default!;

you would use:

public required string FirstName { get; init; }