Conversion from type ‘DBNull’ to type ‘String’ is not valid

i am receiving this problem

Conversion from type ‘DBNull’ to type ‘String’ is not valid.

Line 501: hfSupEmail.Value =
dt.Rows(0)(“SupEmail”)

i am very new to this, i am not really sure what is the exact problem
could someone guide me?

Many thanks

Answers:

Thank you for visiting the Q&A section on Magenaut. Please note that all the answers may not help you solve the issue immediately. So please treat them as advisements. If you found the post helpful (or not), leave a comment & I’ll get back to you as soon as possible.

Method 1

The quick and dirty fix:

hfSupEmail.Value = dt.Rows(0)("SupEmail").ToString()

or for C#:
hfsupEmail.Value = dt.Rows[0]["SupEmail"].ToString();

This works very well when your eventual target and the source data are already strings, because any extra .ToString() call for something that’s already a string is likely to be optimized into a no-op by the jitter, and if it’s NULL the resulting DBNull.Value.ToString() expression produces the empty string you want.

However, if you’re working with non-string types, you may end up doing significant extra work, especially with something like a DateTime or numeric value where you want specific formatting. Remember, internationalization concerns mean parsing and composing date and number values are actually surprisingly expensive operations; doing “extra” work to avoid those operations is often more than worth it.

Method 2

Hope This Help….
dt.Rows(0)("SupEmail") returns null

To avoid this chcek before assigning

If Not IsDBNull(dt.Rows(0)("SupEmail")) Then
    hfSupEmail.Value = dt.Rows(0)("SupEmail")
End If

Method 3

Apparently your dt.Rows(0)("SupEmail") is coming as NULL from DB and you cannot assign NULL to string property. Try replacing that line with:

hfSupEmail.Value = If(IsDbNull(dt.Rows(0)("SupEmail")), String.Empty, dt.Rows(0)("SupEmail").ToString)

The code checks if value is NULL and if it is – replaces it with empty string, otherwise uses original value.

Method 4

You should handle it at DB query level itself.

instead of "select name from student", use "select IsNull(name,'') as name from student"

In this way, DB will handle your NULL value.

Method 5

To handle it from code, here is a small extension method

Imports Microsoft.VisualBasic
Imports System.Runtime.CompilerServices

Public Module HTMLExtensionMethods
    <Extension()> _
    Public Function DefaultIfDBNull(Of T)(ByVal obj As Object) As T
        Return If(Convert.IsDBNull(obj), CType(Nothing, T), CType(obj, T))
    End Function
End Module

Call it like this.
hfSupEmail.Value = dt.Rows(0)("SupEmail").DefaultIfDBNull(Of String)()

Method 6

You can use the Field method of the Datarow combined with an If Operator to check for a Null value in one line like this. If it is null, you can replace it with an empty string (or another string of your choosing):

hfSupEmail.Value = If(dt.Rows(0).Field(Of String)("SupEmail"), "")

Method 7

The easiest way is probably to just concatenate it with an empty string:

hfSupEmail.Value = dt.Rows(0)("SupEmail") & ""

Method 8

        con.Open()
        cmd = New SqlCommand
        cmd.CommandText = " select  sum (Income_Amount)  from Income where Income_Month= '" & ComboBox1.Text & "' and Income_year=" & txtyearpro.Text & ""
        cmd.Connection = con
        dr = cmd.ExecuteReader
        If dr.Read = True Then
            txtincome1.Text = dr(0).ToString  ' ToString  converts null values into string '

        End If


All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0

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