var modeltest1 = db.SysUsers.Where(m => m.State != 0
&& string.IsNullOrEmpty(User_Name) ? true : m.Name.Contains(User_Name));
var modeltest2 = db.SysUsers.Where(m => m.State != 0
&& (string.IsNullOrEmpty(User_Name) ? true : m.Name.Contains(User_Name)));
var modeltest3 = db.SysUsers.Where(m =>
string.IsNullOrEmpty(User_Name) ? true : m.Name.Contains(User_Name)
&& m.State != 0);
var modeltest4 = db.SysUsers.Where(m =>
(string.IsNullOrEmpty(User_Name) ? true : m.Name.Contains(User_Name))
&& m.State != 0);
系统翻译为
Modeltest1
SELECT *
FROM [dbo].[SysUser] AS [Extent1]
WHERE ( CASE WHEN ( ( NOT ( ( 0 = [Extent1].[State] )
AND ( [Extent1].[State] IS NOT NULL )
)
)
AND ( ( @p__linq__0 IS NULL )
OR ( ( CAST(LEN(@p__linq__0) AS INT) ) = 0 )
)
) THEN CAST(1 AS BIT)
WHEN ( [Extent1].[Name] LIKE @p__linq__1 ESCAPE N'~' )
THEN CAST(1 AS BIT)
WHEN ( NOT ( [Extent1].[Name] LIKE @p__linq__1 ESCAPE N'~' )
) THEN CAST(0 AS BIT)
END ) = 1
翻译结果，把第一个条件当作了条件运算符的一部分。 State不等于0且User_Name为空或NULL时，条件成立。
Modeltest2
SELECT *
FROM [dbo].[SysUser] AS [Extent1]
WHERE ( 0 <> [Extent1].[State] )
AND ( ( CASE WHEN ( ( @p__linq__0 IS NULL )
OR ( ( CAST(LEN(@p__linq__0) AS INT) ) = 0 )
) THEN CAST(1 AS BIT)
WHEN ( [Extent1].[Name] LIKE @p__linq__1 ESCAPE N'~' )
THEN CAST(1 AS BIT)
WHEN ( NOT ( [Extent1].[Name] LIKE @p__linq__1 ESCAPE N'~' )
) THEN CAST(0 AS BIT)
END ) = 1 )
结果正常，过滤出State不为0的数据。
Modeltest3
SELECT *
FROM [dbo].[SysUser] AS [Extent1]
WHERE ( CASE WHEN ( ( @p__linq__0 IS NULL )
OR ( ( CAST(LEN(@p__linq__0) AS INT) ) = 0 )
) THEN CAST(1 AS BIT)
WHEN ( ( [Extent1].[Name] LIKE @p__linq__1 ESCAPE N'~' )
AND ( NOT ( ( 0 = [Extent1].[State] )
AND ( [Extent1].[State] IS NOT NULL )
)
)
) THEN CAST(1 AS BIT)
WHEN ( NOT ( ( [Extent1].[Name] LIKE @p__linq__1 ESCAPE N'~' )
AND ( 0 <> [Extent1].[State] )
)
) THEN CAST(0 AS BIT)
END ) = 1
翻译结果，根据条件运行符从右向左结合方向，把第二个条件当作子条件。当User_Name为空或NULL时，均条件成立。
modeltest4
SELECT *
FROM [dbo].[SysUser] AS [Extent1]
WHERE ( ( CASE WHEN ( ( @p__linq__0 IS NULL )
OR ( ( CAST(LEN(@p__linq__0) AS INT) ) = 0 )
) THEN CAST(1 AS BIT)
WHEN ( [Extent1].[Name] LIKE @p__linq__1 ESCAPE N'~' )
THEN CAST(1 AS BIT)
WHEN ( NOT ( [Extent1].[Name] LIKE @p__linq__1 ESCAPE N'~' )
) THEN CAST(0 AS BIT)
END ) = 1 )
AND ( 0 <> [Extent1].[State] )
翻译结果同2
结论：括号是很重要的，不注意的话，结果会千差万别，至于State != 0 ，
一会翻译为 0 <> [Extent1].[State]；
一会翻译为 NOT ( ( 0 = [Extent1].[State] ) AND ( [Extent1].[State] IS NOT NULL ) )
还不清楚是有什么规律，不过看其结果，当其为独立条件时，是翻译为<>的。（至于还有没有例外情况，则未验证）。
呃，题外话，日志格式编排小技巧：
Word里编辑，再原格式粘贴，将色块用表格包装一下，则不会出现分散的情况！