Question

Question:

The below program throws a runtime error (invalid conversion from double) because there is

 + _

followed by a newline and a

 +"

In other words, it’s

"SomeString" + "someotherstring" ++ "yet another string"

 

<STAThread()> _
Sub Main()
    Dim sb As System.Text.StringBuilder = New System.Text.StringBuilder

    Dim dt As New DataTable
    dt.Columns.Add("oeId", GetType(String))
    dt.Columns.Add("ZO_RM_Name", GetType(String))
    dt.Columns.Add("ZO_GB_Name", GetType(String))
    dt.Columns.Add("gubeg", GetType(DateTime))
    dt.Columns.Add("guend", GetType(DateTime))

    Dim dr As DataRow = Nothing

    For i As Integer = 0 To 10 Step 1
        dr = dt.NewRow()
        dr("oeId") = "Organization Unit id " + (i * 1000).ToString()
        dr("ZO_RM_Name") = "Room " + i.ToString()
        dr("ZO_GB_Name") = "Building " + i.ToString()
        dr("gubeg") = System.DateTime.Now
        dr("guend") = System.DateTime.Now.AddDays(22).AddYears(20).AddHours(2)
        dt.Rows.Add(dr)
    Next

    For Each drThisRow As DataRow In dt.Rows
        sb.AppendLine("Organisationseinheit " + drThisRow("oeId").ToString() + _
                               +" in Raum " + drThisRow("ZO_RM_Name").ToString() + " von Gebäude " + drThisRow("ZO_GB_Name").ToString() _
                               + " in der Gültigkeit von " + drThisRow("gubeg").ToString() + " bis " + drThisRow("guend").ToString() + "." _
        )
    Next
End Sub '' Main ''

One could simplify this problem to this one:

 Dim newstring As String = "SomeString" + "someotherstring" + +"yet another string"

It compiles fine, but when the program is run, it throws a runtime error.
Is this a compiler bug ?
Shouldn’t it stop me with a compiler error, like invalid syntax ?

Answer 1

The + operator comes in two variants:

• a binary operator, a + b
• a unary operator, +a

The first one says “take a and add b and produce the results”. The second one says “take a with a positive sign”.

In this case, the expression is evaluated as a string + a double, because VB.NET, with OPTION STRICT OFF will think that you want the second operand here as a string, that will be autoconverted to a double at runtime, because you prefixed it with the unary + operator.

Set OPTION STRICT ON to make VB.NET be more strict with its typechecking at compiletime, at the expense of you having to write code a bit more specific in some cases.