PowerShell and System.Nullable<T>

While helping to answer a question about Exchange cmdlets today, I came across something interesting, which doesn't seem to be very well documented.

A little background, first: in the .NET Framework (starting in Version 2.0), there's a Generic type called System.Nullable<T>. The purpose of this type is to allow you to assign a value of null to Value types (structs, integers, booleans, etc), which are normally not allowed to be null in a .NET application. The Nullable structure consists of two properties: HasValue (a Boolean), and Value (the underlying value type, such as an integer, struct, etc).

A C# method which accepts a Nullable type might look something like this:

int? Multiply(int? operand1, int? operand2)
{
    if (!operand1.HasValue || !operand2.HasValue) { return null; }

    return operand1.Value * operand2.Value;
}

("int?" is C# shorthand for System.Nullable<int> .)

PowerShell appears to do something helpful, though potentially unexpected, when it comes across an instance of System.Nullable: it evaluates to either $null or an object of the underlying type for you, without the need (or the ability) to ever access the HasValue or the Value properties of the Nullable structure yourself:

$variable = [Nullable[int]] 10

$variable.GetType().FullName   # System.Int32

If you assign $null to the Nullable variable instead, the $variable.GetType() line will produce a "You cannot call a method on a null-valued expression" error. You never see the actual System.Nullable structure in your PowerShell code.

What does this have to do with Exchange? Some of the Exchange cmdlets return objects that have public Nullable properties, such as MoveRequestStatistics.BytesTransferred. Going through the MSDN documentation on these classes, you might expect to have to do something like $_.BytesTransferred.Value.ToMB() to get at the ByteQuantifiedSize.ToMB() method, but that won't work. $_.BytesTransferred will either be $null, or it will be an instance of the ByteQuantifiedSize structure; there is no "Value" property in either case. After checking for $null, you'd just do this: $_.BytesTransferred.ToMB()

About the Author

Dave Wyatt

Dave Wyatt is a Microsoft MVP (PowerShell) and a member of PowerShell.org's Board of Directors.

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