CODEWITHSUNDEEP
vb.net
user3267755
user3267755

Reputation: 1070

Conversion from type 'DBNull' to type 'Date' is not valid on tryparse

I am attempting to do a DateTime.TryParse from a date-formatted string, to a new column that I created as part of a DataView table. I am getting this error when the code runs Conversion from type 'DBNull' to type 'Date' is not valid.

This is the line of code:

DateTime.TryParse(dr("IX_ArticleStartDate"), dr("nStartDate"))

And these are the values in my watch when it errors out.

+       dr("IX_ArticleStartDate")   "2015/3/11" {String}    Object
+       dr("nStartDate")    {}  Object

I was under the impression that a TryParse would return a NULL value if it fails to convert datatypes. Is there something that I should be doing different to convert this string to a DateTime datatype?

dr is instantiated as a DataRow

Dim dr As DataRow = dv0.Table.Rows(i)

Upvotes: 2

Views: 3665

Answers (2)

rory.ap
rory.ap

Reputation: 35318

A DateTime is a value type and cannot have a null value. You can do something like this:

Dim result As DateTime

Dim myDate As DateTime? = If(Not dr.IsNull("IX_ArticleStartDate") AndAlso _
    DateTime.TryParse(dr.IsNull("IX_ArticleStartDate").ToString(), result), _
    result, New DateTime?)

In this example, the variable myDate is not actually a DateTime, it's a Nullable(Of DateTime) as indicated by the question mark ? after DateTime in the declaration (see MSDN). The TryParse method actually takes a string value as the first argument and an output parameter which is a DateTime value as the second argument. If the parse is successful, it returns True and sets the output parameter to the parsed DateTime value; on the other hand, if the parse is not successful, it returns False and sets the output parameter to DateTime.MinDate which is not very useful because it's difficult to distinguish between a valid use of DateTime.MinDate and null.

The example makes use of a ternary operator which either parses and returns the date value, if it's not null, or else returns a nullable date instead (New DateTime?).

You would then use myDate.HasValue() accordingly to determine if the value is null, and if it's not null, you can use its value: myDate.Value.

Upvotes: 2

Jürgen Steinblock
Jürgen Steinblock

Reputation: 31743

VB implicitly try's to cast the DBNull Value to DateTime, since the method signature of DateTime.TryParse is

Public Shared Function TryParse(s As String, ByRef result As Date) As Boolean

which fails. You can use a variable instead:

dim startDate as DateTime
If DateTime.TryParse(dr("IX_ArticleStartDate").ToString(), startDate) Then
    dr("startDate") = startDate
End If

Upvotes: 2

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