Why does the setter of a VB.NET property require a typed argument and why is it ByVal?

Question

In C#, a property's setter value keyword will automatically be same as the property's type.

For example, in C# ,type of value is string

private string str = string.Empty;
public string MyText 
{
    get { return str; }
    set { str = value; }
}

If we convert this snippet to VB.Net we get

Private str As String = String.Empty
Public Property MyText() As String
    Get
        Return str 
    End Get
    Set(ByVal value As String)
        str = value
    End Set
End Property

Questions

Why does set have this line Set(ByVal value As String)? Shouldn't value type automatically be String. This way.

Private str As String = String.Empty
Public Property MyText() As String
    Get
        Return str 
    End Get
    Set
        str = value
    End Set
End Property

What's the use of that?

I cannot change BYVal to ByRef (I tried, it gives error), then what's use of that also?

Good question, not sure why it was given a -1 straight off the bat. — Alastair Pitts, Sep 12 '12 at 07:35

score 9 · Accepted Answer · edited Sep 12 '12 at 16:25

9

You can omit the (ByVal value As String) part. Visual Studio will keep adding it, but it is not required by either the language nor the compiler.

You can use a parameter name other than value. Also note that since VS2010 SP1, you can omit the ByVal keyword.

Example:

Class Test

    Private str As String = String.Empty

    Public Property MyText() As String
        Get
            Return str 
        End Get
        Set
            str = value
        End Set
    End Property

    Public Property MyText2() As String
        Get
            Return str 
        End Get
        Set(something As String)
            str = something
        End Set
    End Property

End Class

edited Sep 12 '12 at 16:25

cuongle

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answered Sep 12 '12 at 07:37

sloth

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Hey if I delete it, as you said, Visual Studio automatically adds it. – Nikhil Agrawal Sep 12 '12 at 07:39
@NikhilAgrawal - that the VS editor insisting on adding it (similarly it will add back `ByVal` if you remove it). But the Visual Basic *language* doesn't require these things. – Damien_The_Unbeliever Sep 12 '12 at 07:41
@NikhilAgrawal It doesn't mean that the compiler needs it. – sloth Sep 12 '12 at 07:42
@Damien_The_Unbeliever *(similarly it will add back ByVal if you remove it)*: It won't do this anymore since VS2010 SP1. – sloth Sep 12 '12 at 07:43
@BigYellowCactus - ah. When we started using VS2010, we started migrating from VB -> C# for new development, so I'm using it less and less with the newer VSs – Damien_The_Unbeliever Sep 12 '12 at 07:46
Nor do I, so I've +1ed it. I've added the relevant quotes from the VB language spec to my answer. – Damien_The_Unbeliever Sep 12 '12 at 07:52

score 2 · Answer 2 · answered Sep 12 '12 at 07:30

I think, its part of the syntax in Visual basic. You can omit the data type , if OptionStrict is not on. But if you specify the type, it must be same as of property type.

From MSDN - Set Statement (Visual Basic)

datatype
Required if Option Strict is On. Data type of the value parameter. The data type specified must be the same as the data type of the property where this Set statement is declared.

Damien_The_Unbeliever · Answer 3 · 2012-09-12T07:53:29.343

Because you are allowed in VB to change the name of the parameter:

Public Property MyText() As String
    Get
        Return str
    End Get
    Set(ByVal val As String)
        str = val
    End Set
End Property

You can't change the ByVal to ByRef because property parameters can only ever be passed by value. But VB sometimes favours stating facts that are unalterable.

The developers could (in theory) have let you just have:

Set(val)

Because nothing else is alterable. But they opted to make the parameters for properties resembles the parameters for Functions and Subs.

In fact, you don't have to specify the parameter at all. Per the VB language spec (9.7.2):

If a parameter list is specified, it must have one member, that member must have no modifiers except ByVal, and its type must be the same as the type of the property. The parameter represents the property value being set. If the parameter is omitted, a parameter named Value is implicitly declared

And (9.5.2):

A parameter that does not specify ByRef or ByVal defaults to ByVal.

score 1 · Answer 4 · answered Sep 12 '12 at 07:38

1

If the value were ByRef, the Setter would be able to modify the caller's value. It doesn't make sense to allow a property setter to cause side effects like this.

answered Sep 12 '12 at 07:38

Joe

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score 0 · Answer 5 · answered Sep 12 '12 at 17:22

0

You could have value be an enum and do a select case on it for instance and set the value that way. You don't have to limit the property's set call to only passing the same value type which is a nice feature

answered Sep 12 '12 at 17:22

Darren Grantham

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Why does the setter of a VB.NET property require a typed argument and why is it ByVal?

5 Answers5