CODEWITHSUNDEEP
c#datarow
that0th3rGuy
that0th3rGuy

Reputation: 1386

DataRow.Field<T>(string Column) throws invalid cast exception

Good day,

IDE Visual Studio 2010
.NET 3.5
Platform WinForms

The SO question " difference between getting value from DataRow " refers.

I have a database table with a column [ID] INT IDENTITY(1, 1) NOT NULL PRIMARY KEY. When querying this table and storing the values in local variables I get an invalid cast exception; sample code:

string sQuery = @"
    SELECT [ID], [Description]
    FROM [Sources]
    ORDER BY [Description] ";

using (DataTable dtSources = SQLHelper.Fetch(sQuery))
{
    foreach (DataRow drSource in dtSources.Rows)
    {
        int iID = drSource.Field<int>("ID"); // InvalidCastException
        string sDescrption = drSource.Field<string>("Description");
    }
}

When stepping the execution and performing a "Quick Watch" on the faulty line I discovered that, by changing the line to drSource.Field<object>("ID"), the cell value type is that of short and not of int. Why would this happen when, in the table definition, this is clearly and int? Furthermore, short should be implicitly converted to int since short is smaller and should "fit" right?

Upvotes: 11

Views: 47851

Answers (5)

Karen
Karen

Reputation: 111

If your column is a nullable int, but you are trying to assign to an int, with a default value of 0:

using (DataTable dtSources = SQLHelper.Fetch(sQuery))
{ 
    foreach (DataRow drSource in dtSources.Rows)'
    { 
        int iID = drSource.Field<int?>("ID") ?? 0; 
        string sDescrption = drSource.Field<string>("Description"); 
    }
}

If your column is a nullable int and you want to assign to a nullable int:

using (DataTable dtSources = SQLHelper.Fetch(sQuery))
{
    foreach (DataRow drSource in dtSources.Rows)
    {
        int? iID = drSource.Field<int?>("ID");
        string sDescrption = drSource.Field<string>("Description"); 
    }
}

Upvotes: 11

Kendon Darlington
Kendon Darlington

Reputation: 153

Another way to get your row value into an int variable is to use 'object' and convert it to Int32 like so:

int iID = Convert.ToInt32(row.Field<object>("ID"));

Upvotes: 2

Himalaya Garg
Himalaya Garg

Reputation: 1609

Agree with Richard's answer just better to use, 

int iID = Convert.ToInt32(drSource["ID"]);

Upvotes: 9

Sergey Berezovskiy
Sergey Berezovskiy

Reputation: 236218

According to implementation of Field extension, your field has DbNull value.

public static T Field<T>(this DataRow row, string columnName)
    {
        DataSetUtil.CheckArgumentNull<DataRow>(row, "row");
        return UnboxT<T>.Unbox(row[columnName]);
    }

UnboxT is a private class that provides methods for converting object to T. In your case ValueField converter is used:

private static class UnboxT<T>
{
    internal static readonly Converter<object, T> Unbox;

    static UnboxT()
    {
       DataRowExtensions.UnboxT<T>.Unbox =  
          new Converter<object, T>(DataRowExtensions.UnboxT<T>.ValueField);
    }

    private static T ValueField(object value)
    {
        if (DBNull.Value == value)
        {
            // You get this exception 
            throw DataSetUtil.InvalidCast(Strings.DataSetLinq_NonNullableCast(typeof(T).ToString()));
        }
        return (T) value;
    }
}

Upvotes: 5

Richard
Richard

Reputation: 8280

Out of curiosity, what happens if you explicitly cast it yourself from the key/value collection?

int iID = (int)drSource["ID"];

Upvotes: 5

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