Question

I’ve got this function:

Public Shared Function GetQueryStringValue(Of T As Structure)(ByVal queryStringVariable As String) As T
        Dim queryStringObject As Nullable(Of T) = Nothing
        If queryStringVariable <> Nothing Then
            If HttpContext.Current.Request.QueryString(queryStringVariable) IsNot Nothing Then
                queryStringObject = DirectCast(HttpContext.Current.Request.QueryString(queryStringVariable), T)
            End If
        End If

        Return queryStringObject
End Function

Which I was hoping to call like this:

Dim userId As Integer = SessionUtil.GetSessionValue(Of Integer)("uid")

I was trying to make it generic since in the end a query string value could be at least an integer or a string, but possibly also a double and others. But I get the error:

Value of 'String' cannot be converted to Type 'T'

I did this exact same thing with Session variables and it worked. Anyone know a way to make this work?

EDIT: Jonathan Allen below has a more simpler answer using CObj() or CTypeDynamic(). But the below also works from Convert string to nullable type (int, double, etc…)

Dim conv As TypeConverter = TypeDescriptor.GetConverter(GetType(T))
queryStringObject = DirectCast(conv.ConvertFrom(queryStringVariable), T)

Answer 1

The safest way is to use CTypeDynamic. This will ensure that implicit/explicit operators are used.

Function Convert(Of T)(s As String) As T
    Return Microsoft.VisualBasic.CTypeDynamic(Of T)(s)
End Function

This will work for simple types but will fail for complex ones.

Function Convert(Of T)(s As String) As T
    Return CType(CObj(s), T)
End Function