InvalidCastException

Question

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

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.

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References:

• Cast from string "<string>" to type '<typename>' is not valid
• Cast from type '<typename1>' to type '<typename2>' is not valid
• System.InvalidCastException class
• DBNull.IConvertible.ToType(Type, IFormatProvider) Method
• DBNull.IConvertible.ToChar(IFormatProvider) Method
• Type conversion in .NET
• Casting and type conversions (C# Programming Guide)