22

我有一个 sql 查询，它的参数在数据库（Sql Server）中可以为空。更新方法工作正常，直到该用户在字段中输入一个空白，这会为 DataTime 对象生成一个空值（该对象可以为空）。问题是当dbCommand.ExecuteNonQuery();.

这是我为该字段构建参数的方式：

    IDataParameter dbParam_au_id = new SqlParameter();
    dbParam_au_id.ParameterName = "@birthday";
    dbParam_au_id.Value = birthday;
    dbParam_au_id.DbType = DbType.DateTime;
    dbCommand.Parameters.Add(dbParam_au_id);

我尝试将生日的空值转换为 DBNull.Value ：

    IDataParameter dbParam_au_id = new SqlParameter();
    dbParam_au_id.ParameterName = "@birthday";
    dbParam_au_id.Value = birthday??DBNull.Value;
    dbParam_au_id.DbType = DbType.DateTime;
    dbCommand.Parameters.Add(dbParam_au_id);

但是这段代码不会编译，我得到错误：

错误 1 ​​运算符 '??' 不能应用于“System.DateTime？”类型的操作数？和'System.DBNull'

任何的想法？

4

2 回答 2

57

类型不兼容。尝试这样的事情：

dbParam_au_id.Value = (object)birthday ?? DBNull.Value;
于 2009-05-14T13:35:10.080 回答
15

If the SqlParameter class was written correctly the first time... a C# null value would be handled as DBNull.Value. That would be intuitive, so OF COURSE setting an SqlParameter value to null is functionally equivalent to removing it from the SqlParameterCollection.

To correct this ridiculous API design error, create your own AddParameter method (with overloads), which takes a SqlParameterCollection, a String (parameter name), and an Object (parameter value).

#region Add by Name/Value.
/// <summary>
/// Adds an input parameter with a name and value.  Automatically handles conversion of null object values to DBNull.Value.
/// </summary>
/// <param name="parameters">SqlParameterCollection from an SqlCommand instance.</param>
/// <param name="name">The name of the parameter to add.</param>
/// <param name="value">The value of the parameter to add.</param>
private static void AddParameter( SqlParameterCollection parameters, string name, object value )
{
    parameters.Add( new SqlParameter( name, value ?? DBNull.Value ) );
}

/// <summary>
/// Adds a parameter with a name and value.  You specify the input/output direction.  Automatically handles conversion of null object values to DBNull.Value.
/// </summary>
/// <param name="parameters">SqlParameterCollection from an SqlCommand instance.</param>
/// <param name="name">The name of the parameter to add.</param>
/// <param name="value">The value of the parameter to add.  If null, this is automatically converted to DBNull.Value.</param>
/// <param name="direction">The ParameterDirection of the parameter to add (input, output, input/output, or return value).</param>
private static void AddParameter( SqlParameterCollection parameters, string name, object value, ParameterDirection direction )
{
    SqlParameter parameter = new SqlParameter( name, value ?? DBNull.Value );
    parameter.Direction = direction;
    parameters.Add( parameter );
}
#endregion

#region Add by Name, Type, and Value.
/// <summary>
/// Adds an input parameter with a name, type, and value.  Automatically handles conversion of null object values to DBNull.Value.
/// </summary>
/// <param name="parameters">SqlParameterCollection from an SqlCommand instance.</param>
/// <param name="name">The name of the parameter to add.</param>
/// <param name="type">Specifies the SqlDbType of the parameter.</param>
/// <param name="value">The value of the parameter to add.  If null, this is automatically converted to DBNull.Value.</param>
private static void AddParameter( SqlParameterCollection parameters, string name, SqlDbType type, object value )
{
    AddParameter( parameters, name, type, 0, value ?? DBNull.Value, ParameterDirection.Input );
}

/// <summary>
/// Adds a parameter with a name, type, and value.  You specify the input/output direction.  Automatically handles conversion of null object values to DBNull.Value.
/// </summary>
/// <param name="parameters">SqlParameterCollection from an SqlCommand instance.</param>
/// <param name="name">The name of the parameter to add.</param>
/// <param name="type">Specifies the SqlDbType of the parameter.</param>
/// <param name="value">The value of the parameter to add.  If null, this is automatically converted to DBNull.Value.</param>
/// <param name="direction">The ParameterDirection of the parameter to add (input, output, input/output, or return value).</param>
private static void AddParameter( SqlParameterCollection parameters, string name, SqlDbType type, object value, ParameterDirection direction )
{
    AddParameter( parameters, name, type, 0, value ?? DBNull.Value, direction );
}
#endregion

#region Add by Name, Type, Size, and Value.
/// <summary>
/// Adds an input parameter with a name, type, size, and value.  Automatically handles conversion of null object values to DBNull.Value.
/// </summary>
/// <param name="parameters">SqlParameterCollection from an SqlCommand instance.</param>
/// <param name="name">The name of the parameter to add.</param>
/// <param name="type">Specifies the SqlDbType of the parameter.</param>
/// <param name="size">Specifies the size of the parameter for parameter types of variable size.  Set to zero to use the default size.</param>
/// <param name="value">The value of the parameter to add.  If null, this is automatically converted to DBNull.Value.</param>
private static void AddParameter( SqlParameterCollection parameters, string name, SqlDbType type, int size, object value )
{
    AddParameter( parameters, name, type, size, value ?? DBNull.Value, ParameterDirection.Input );
}

/// <summary>
/// Adds a parameter with a name, type, size, and value.  You specify the input/output direction.  Automatically handles conversion of null object values to DBNull.Value.
/// </summary>
/// <param name="parameters">SqlParameterCollection from an SqlCommand instance.</param>
/// <param name="name">The name of the parameter to add.</param>
/// <param name="type">Specifies the SqlDbType of the parameter.</param>
/// <param name="size">Specifies the size of the parameter for parameter types of variable size.  Set to zero to use the default size.</param>
/// <param name="value">The value of the parameter to add.  If null, this is automatically converted to DBNull.Value.</param>
/// <param name="direction">The ParameterDirection of the parameter to add (input, output, input/output, or return value).</param>
private static void AddParameter( SqlParameterCollection parameters, string name, SqlDbType type, int size, object value, ParameterDirection direction )
{
    SqlParameter parameter;
    if (size < 1)
        parameter = new SqlParameter( name, type );
    else
        parameter = new SqlParameter( name, type, size );
    parameter.Value = value ?? DBNull.Value;
    parameter.Direction = direction;
    parameters.Add( parameter );
}
#endregion

As you can see, inside that method (and overloads), where the value is already typed as an object, I use the "value ?? DBNull.Value" statement to enforce the null = DBNull.Value rule.

Now, when you pass null object references or nullable types with no values to your AddParameter method, you get the expected, intuitive behavior, where a DBNull.Value is passed to the query.

I can't imagine why the API was implemented as it is, because if I wanted a parameter to be ignored, I would not ADD it and then SET it's value to null. I would either NOT add it in the first place, or I would REMOVE it from the SqlParameterCollection. If I ADD a parameter, and SET it's value (even if set to null), I expect it to be USED in the query, I expect null to mean null value.

I've heard they didn't implement it the "correct" way for performance reasons, but that's ridiculous, as demonstrated, because calling SqlParameterCollection.AddWithValue method converts everything to an object anyway, and converting a Nullable instance with no value to a null object is an intrinsic part of the C# language which is not a performance hit at all. Microsoft should really fix this.

于 2009-09-14T22:28:24.057 回答