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new SqlParameter("@name", SqlDbType.Varchar) { Value = name };
根据MSDN解释:如果未在 size 参数中显式设置 Size,则从 dbType 参数的值推断出该大小。string sql = "select top 1 * from tb where name = @o";
var para = new SqlParameter("@o", SqlDbType.VarChar) {Value = "ab"};
SqlHelper.ExecuteReader(ReadConnectionString, CommandType.Text, sql, para);
通过SqlProfiler捕获到的sql如下:exec sp_executesql N'select top 1 * from tb where name = @o',N'@o nvarchar(2)',@o=N'ab'
如果把参数的值改成abcd,则通过SqlProfiler捕获到的sql如下:exec sp_executesql N'select top 1 * from tb where name = @o',N'@o nvarchar(4)',@o=N'abcd'
通过下面的sql,可以看出执行计划是否有重用:--先清空执行计划缓存
DBCC FREESYSTEMCACHE ('ALL')
DBCC FREEPROCCACHE
GO
SELECT * FROM sys.dm_exec_cached_plans WHERE cacheobjtype = 'Compiled Plan'
GO
exec sp_executesql N'select top 1 * from tb where name = @o',N'@o nvarchar(2)',@o=N'ab'
GO
SELECT * FROM sys.dm_exec_cached_plans WHERE cacheobjtype = 'Compiled Plan'
GO
exec sp_executesql N'select top 1 * from tb where name = @o',N'@o nvarchar(4)',@o=N'abcd'
GO
SELECT * FROM sys.dm_exec_cached_plans WHERE cacheobjtype = 'Compiled Plan'
GO
new SqlParameter("@name", SqlDbType.Varchar, 4000) { Value = name };
如果是Int、Float、Bigint之类的参数,可以不用指定size属性