InvalidCastException

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InvalidCastException

Corey H 0

How to fix [InvalidCastException: Conversion from type 'DBNull' to type 'String' is not valid.]

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2 answers

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Answer 1

InvalidCastException: Conversion from type 'DBNull' to type 'String' is not valid occurs because code is trying to cast or convert a database null (DBNull.Value) directly to String, which is not a supported conversion.

To fix it, always check for DBNull before converting, and provide a replacement value when the database field is null. Typical VB patterns:

' Example: reading from a DataRow or DataReader
Dim value As Object = row("SomeColumn")   ' or reader("SomeColumn")

Dim text As String
If value Is DBNull.Value Then
    text = String.Empty   ' or a default like "N/A"
Else
    text = CStr(value)    ' safe because it is not DBNull
End If

Or using IIf/If for brevity:

Dim text As String = If(IsDBNull(row("SomeColumn")), String.Empty, CStr(row("SomeColumn")))

Key points:

DBNull is a special type used by ADO.NET to represent database NULL; it cannot be cast directly to String.
Conversions that are not supported throw InvalidCastException and should be fixed in code rather than caught and ignored.
Use Convert or CStr only after confirming the value is not DBNull.

References:

Answer 2

Hi @Corey H ,

Thanks for reaching out.

This would happen when the value coming back from the database is actually NULL. In .NET, that comes through as DBNull.Value, which represents a database null and cannot be cast directly to a String.

The appropriate approach is to check for DBNull before assigning the value to a string, then use a safe default such as an empty string if the database field is null.

For example, if you are reading from a DataRow, you can do this:

Dim value As Object = row("SomeColumn")
Dim text As String

If value Is DBNull.Value Then
  text = String.Empty
Else
  text = CStr(value)
End If

Or, if you prefer a shorter version:

Dim value As Object = row("SomeColumn")
Dim text As String = If(IsDBNull(value), String.Empty, CStr(value))

If you are using a DataReader, this pattern is usually cleaner:

Dim columnIndex As Integer = reader.GetOrdinal("SomeColumn")
Dim text As String

If reader.IsDBNull(columnIndex) Then
  text = String.Empty
Else
  text = reader.GetString(columnIndex)
End If

You can replace String.Empty with another fallback value, such as "N/A", if that makes more sense for your application.

Hope this helps! If my explanation and the information I provided were helpful, I would greatly appreciate it if you could follow the instructions here so others with the same problem can benefit as well.

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InvalidCastException

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