i am receiving this problem
Conversion from type ‘DBNull’ to type ‘String’ is not valid.
Line 501: hfSupEmail.Value =
i am very new to this, i am not really sure what is the exact problem
could someone guide me?
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The quick and dirty fix:
hfSupEmail.Value = dt.Rows(0)("SupEmail").ToString()
or for C#:
hfsupEmail.Value = dt.Rows["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.
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
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.
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.
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)()
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"), "")
The easiest way is probably to just concatenate it with an empty string:
hfSupEmail.Value = dt.Rows(0)("SupEmail") & ""
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