DataRow.Field <T> （字符串列）抛出无效的强制转换异常

Question

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?

Answer 1

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"); 
    }
}

Answer 2

更好地同意Richard的回答，更好地使用它，

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

Answer 3

出于好奇，如果您自己从键/值集合中明确地将其强制转换会怎样？

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

Answer 4

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;
    }
}

Answer 5

将行值转换为int变量的另一种方法是使用'object'并将其转换为Int32，如下所示：

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