Question

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What do two question marks together mean in C#?

Hi, I was looking for some trainings of MVC 2 in C# and I found this sintax:

ViewData["something"] = something ?? true;

So, what is that ‘??’ means ?.

Answer 1

It’s the null-coalescing operator.

It returns the first argument unless it is null, in which case it returns the second.

x ?? y is roughly equivalent to this (except that the first argument is only evaluated once):

if (x == null)
{
     result = y;
}
else
{
     result = x;
}

Or alternatively:

(x == null) ? y : x

It is useful for providing a default value for when a value can be null:

Color color = user.FavouriteColor ?? defaultColor;

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COALESCE

When used in a LINQ to SQL query the ?? operator can be translated to a call to COALESCE. For example this LINQ query:

var query = dataContext.Table1.Select(x => x.Col1 ?? "default");

can result in this SQL query:

SELECT COALESCE([t0].[col1],@p0) AS [value]
FROM [dbo].[table1] AS [t0]