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InvalidCastException

Corey H 0 Reputation points
2026-05-31T13:55:18.6366667+00:00

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

Developer technologies | VB
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  1. AI answer

    2026-05-31T13:55:27.5366667+00:00

    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:

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  2. Jack Dang (WICLOUD CORPORATION) 18,800 Reputation points Microsoft External Staff Moderator
    2026-06-01T03:28:31.7033333+00:00

    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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