Non-nullable default return null warning

Question

In C#8 we can now enable nullables, which means that reference types by default are considered not null by the compiler unless explicitly declared as nullable. Yet, it seems the compiler still throws a warning when trying to return a default generic with the notnull constraint. Consider the following example:

public TReturn TestMethod<TReturn>() where TReturn : notnull
{
    return default; // default is flagged with a compiler warning for possible null reference return
}

I thought maybe it would help if I also enforced that the return type must have an empty constructor, but it produces the same result:

public TReturn TestMethod<TReturn>() where TReturn : notnull, new()
{
    return default; // default is flagged with a compiler warning for possible null reference return
}

Why is the compiler flagging this line?

I’m pretty certain default will always return null for classes. It won’t attempt to construct a new instance, even if there’s a default constructor. — Jeremy Caney
– Jeremy Caney, Commented May 20, 2020 at 18:29
@HotN: With the new() constraint in place, you could instead call return new TReturn(). — Jeremy Caney
– Jeremy Caney, Commented May 20, 2020 at 18:34
@JeremyCaney "parameterless" means "has no parameters". "empty" means "has no body". I suspect that OP meant the former. — D Stanley
– D Stanley, Commented May 20, 2020 at 18:34

D Stanley · Accepted Answer · 2020-05-20 19:47:25Z

2

TReturn : notnull means that TReturn must be a non-nullable type (which can be either a value type or a non-nullable reference type). Unfortunately, the value of default for non-nullable reference types is still null, hence the compiler warning.

If you want the "default" for a non-nullable reference type to be whatever is created with a parameterless constructor, for example, you could do:

public TReturn TestMethod<TReturn>() where TReturn : notnull, new()
{
    return new TReturn(); 
}

edited May 20, 2020 at 19:47

answered May 20, 2020 at 18:49

D Stanley

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4 Comments

Jeremy Caney Over a year ago

Why not just use return new TReturn() with the ‘new()` constraint? That guarantees a parameterless constructor, and is simpler syntactically.

D Stanley Over a year ago

@JeremyCaney I had thought new() only worked for reference types - I was wrong...

jeancallisti Over a year ago

Will this work if TReturn is a Guid? I'm worried that it won't be Guid.Empty.

Eric J. Over a year ago

This won't work if TReturn is string since it doesn't have a parameterless constructor.

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