从“DBNull”类型到“String”类型的转换无效

Question

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

Answer 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.

Answer 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

Answer 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.

Answer 4

You should handle it at DB query level itself.

instead of "select name from student", use "select  from student"

In this way, DB will handle your NULL value.

Answer 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)()

Answer 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"), "")

Answer 7

最简单的方法可能是将它与一个空字符串连接起来：

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

Answer 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