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  • Entity Framework 第四篇 优化SQL查询

    Expression<Func<TEntity, bool>>与Func<TEntity, bool>的异同
      public IList<TEntity> Search<TEntity>(Expression<Func<TEntity, bool>> predicate = null) where TEntity : class
            {
                if (predicate == null)
                {
                    return dbContext.Set<TEntity>().ToList();
                }
                else
                {
                    return dbContext.Set<TEntity>().Where(predicate).ToList();
                }
    
            }

    本人通过跟踪sql语句得知

    predicate如果为委托类型的时候,dbContext.Set<TEntity>().Where(predicate).ToList();对应的sql语句是

    SELECT 
        [Extent1].[UserID] AS [UserID], 
        [Extent1].[LoginName] AS [LoginName], 
        [Extent1].[Password] AS [Password], 
        [Extent1].[UserName] AS [UserName], 
        [Extent1].[UserType] AS [UserType], 
        [Extent1].[Tel] AS [Tel], 
        [Extent1].[CreateDate] AS [CreateDate], 
        [Extent1].[LoginNum] AS [LoginNum], 
        [Extent1].[LastDate] AS [LastDate], 
        [Extent1].[ShopID] AS [ShopID]
        FROM [dbo].[S_Users] AS [Extent1]
       

    但是当predicate如果为表达式树的时候,语句如下,

    SELECT 
        [Extent1].[UserID] AS [UserID], 
        [Extent1].[LoginName] AS [LoginName], 
        [Extent1].[Password] AS [Password], 
        [Extent1].[UserName] AS [UserName], 
        [Extent1].[UserType] AS [UserType], 
        [Extent1].[Tel] AS [Tel], 
        [Extent1].[CreateDate] AS [CreateDate], 
        [Extent1].[LoginNum] AS [LoginNum], 
        [Extent1].[LastDate] AS [LastDate], 
        [Extent1].[ShopID] AS [ShopID]
        FROM [dbo].[S_Users] AS [Extent1]
        WHERE N'liuyangh' = [Extent1].[LoginName]

    所以我们尽量用表达式树传参,而不是单纯地lambda表达式或者委托类型。

    为什么会这样呢?

    因为dbContext.Set<TEntity>().Where(predicate),当是表达式树的时候返回的是IQueryable<TSource>

    public static IQueryable<TSource> Where<TSource>(this IQueryable<TSource> source, Expression<Func<TSource, bool>> predicate);

      如果是委托类型返回的是IEnumerable<TSource>
    public static IEnumerable<TSource> Where<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate);

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  • 原文地址:https://www.cnblogs.com/njcxwz/p/5581388.html
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