C# Where（）.Count（）和Count（）之间的差异

以上面的例子为例，我列出了30岁以上的员工名单

方法1和方法2的区别是什么？

---

你喜欢哪一种？为什么？

为了可读性，我更喜欢第二种方法。如果您查看生成的sql代码，它是相同的

方法1:

using (DBEntities db = new DBEntities())
{
   var employeeAgedAbove30 = db.Employees.Where(s => s.Age > 30).Count(); // Method 1

   employeeAgedAbove30 = db.Employees.Count(s => s.Age > 30); // Method 2

}

-- Region Parameters
DECLARE @p0 Int = 30
-- EndRegion
SELECT COUNT(*) AS [value]
FROM [tblEmployees] AS [t0]
WHERE [t0].[Age] > @p0
GO

-- Region Parameters
DECLARE @p0 Int = 30
-- EndRegion
SELECT COUNT(*) AS [value]
FROM [tblEmployees] AS [t0]
WHERE [t0].[Age] > @p0

IL_0001:  ldarg.0     
IL_0002:  stloc.0     // db
IL_0003:  ldloc.0     // db
IL_0004:  callvirt    LINQPad.User.TypedDataContext.get_TblEmployees
IL_0009:  ldtoken     LINQPad.User.TblEmployees
IL_000E:  call        System.Type.GetTypeFromHandle
IL_0013:  ldstr       "t"
IL_0018:  call        System.Linq.Expressions.Expression.Parameter
IL_001D:  stloc.1     // CS$0$0000
IL_001E:  ldloc.1     // CS$0$0000
IL_001F:  ldtoken     LINQPad.User.TblEmployees.get_Age
IL_0024:  call        System.Reflection.MethodBase.GetMethodFromHandle
IL_0029:  castclass   System.Reflection.MethodInfo
IL_002E:  call        System.Linq.Expressions.Expression.Property
IL_0033:  ldc.i4.s    1E 
IL_0035:  box         System.Int32
IL_003A:  ldtoken     System.Int32
IL_003F:  call        System.Type.GetTypeFromHandle
IL_0044:  call        System.Linq.Expressions.Expression.Constant
IL_0049:  ldtoken     System.Nullable<System.Int32>
IL_004E:  call        System.Type.GetTypeFromHandle
IL_0053:  call        System.Linq.Expressions.Expression.Convert
IL_0058:  call        System.Linq.Expressions.Expression.GreaterThan
IL_005D:  ldc.i4.1    
IL_005E:  newarr      System.Linq.Expressions.ParameterExpression
IL_0063:  stloc.2     // CS$0$0001
IL_0064:  ldloc.2     // CS$0$0001
IL_0065:  ldc.i4.0    
IL_0066:  ldloc.1     // CS$0$0000
IL_0067:  stelem.ref  
IL_0068:  ldloc.2     // CS$0$0001
IL_0069:  call        System.Linq.Expressions.Expression.Lambda
IL_006E:  call        System.Linq.Queryable.Where
IL_0073:  call        System.Linq.Queryable.Count

00:00:00.0019229
00:00:00.0007023

-- Region Parameters
DECLARE @p0 Int = 1
-- EndRegion
SELECT COUNT(*) AS [value]
FROM [Employees] AS [t0]
WHERE [t0].[Type] = @p0

SQL

db.TblEmployees.Where (t =>t.Age>30).Count ()

db.TblEmployees.Count (t =>t.Age>30)

方法2:

using (DBEntities db = new DBEntities())
{
   var employeeAgedAbove30 = db.Employees.Where(s => s.Age > 30).Count(); // Method 1

   employeeAgedAbove30 = db.Employees.Count(s => s.Age > 30); // Method 2

}

-- Region Parameters
DECLARE @p0 Int = 30
-- EndRegion
SELECT COUNT(*) AS [value]
FROM [tblEmployees] AS [t0]
WHERE [t0].[Age] > @p0
GO

-- Region Parameters
DECLARE @p0 Int = 30
-- EndRegion
SELECT COUNT(*) AS [value]
FROM [tblEmployees] AS [t0]
WHERE [t0].[Age] > @p0

IL_0001:  ldarg.0     
IL_0002:  stloc.0     // db
IL_0003:  ldloc.0     // db
IL_0004:  callvirt    LINQPad.User.TypedDataContext.get_TblEmployees
IL_0009:  ldtoken     LINQPad.User.TblEmployees
IL_000E:  call        System.Type.GetTypeFromHandle
IL_0013:  ldstr       "t"
IL_0018:  call        System.Linq.Expressions.Expression.Parameter
IL_001D:  stloc.1     // CS$0$0000
IL_001E:  ldloc.1     // CS$0$0000
IL_001F:  ldtoken     LINQPad.User.TblEmployees.get_Age
IL_0024:  call        System.Reflection.MethodBase.GetMethodFromHandle
IL_0029:  castclass   System.Reflection.MethodInfo
IL_002E:  call        System.Linq.Expressions.Expression.Property
IL_0033:  ldc.i4.s    1E 
IL_0035:  box         System.Int32
IL_003A:  ldtoken     System.Int32
IL_003F:  call        System.Type.GetTypeFromHandle
IL_0044:  call        System.Linq.Expressions.Expression.Constant
IL_0049:  ldtoken     System.Nullable<System.Int32>
IL_004E:  call        System.Type.GetTypeFromHandle
IL_0053:  call        System.Linq.Expressions.Expression.Convert
IL_0058:  call        System.Linq.Expressions.Expression.GreaterThan
IL_005D:  ldc.i4.1    
IL_005E:  newarr      System.Linq.Expressions.ParameterExpression
IL_0063:  stloc.2     // CS$0$0001
IL_0064:  ldloc.2     // CS$0$0001
IL_0065:  ldc.i4.0    
IL_0066:  ldloc.1     // CS$0$0000
IL_0067:  stelem.ref  
IL_0068:  ldloc.2     // CS$0$0001
IL_0069:  call        System.Linq.Expressions.Expression.Lambda
IL_006E:  call        System.Linq.Queryable.Where
IL_0073:  call        System.Linq.Queryable.Count

00:00:00.0019229
00:00:00.0007023

-- Region Parameters
DECLARE @p0 Int = 1
-- EndRegion
SELECT COUNT(*) AS [value]
FROM [Employees] AS [t0]
WHERE [t0].[Type] = @p0

SQL

db.TblEmployees.Where (t =>t.Age>30).Count ()

db.TblEmployees.Count (t =>t.Age>30)

归根结底，这更多的是个人偏好。对于不习惯linq的人来说，在计数之前使用where子句可能更具可读性

编辑1

在旁注上。是的，生成sql是相同的。但是IL代码在一个地方是不同的。应用计数和where时

方法1:

using (DBEntities db = new DBEntities())
{
   var employeeAgedAbove30 = db.Employees.Where(s => s.Age > 30).Count(); // Method 1

   employeeAgedAbove30 = db.Employees.Count(s => s.Age > 30); // Method 2

}

-- Region Parameters
DECLARE @p0 Int = 30
-- EndRegion
SELECT COUNT(*) AS [value]
FROM [tblEmployees] AS [t0]
WHERE [t0].[Age] > @p0
GO

-- Region Parameters
DECLARE @p0 Int = 30
-- EndRegion
SELECT COUNT(*) AS [value]
FROM [tblEmployees] AS [t0]
WHERE [t0].[Age] > @p0

IL_0001:  ldarg.0     
IL_0002:  stloc.0     // db
IL_0003:  ldloc.0     // db
IL_0004:  callvirt    LINQPad.User.TypedDataContext.get_TblEmployees
IL_0009:  ldtoken     LINQPad.User.TblEmployees
IL_000E:  call        System.Type.GetTypeFromHandle
IL_0013:  ldstr       "t"
IL_0018:  call        System.Linq.Expressions.Expression.Parameter
IL_001D:  stloc.1     // CS$0$0000
IL_001E:  ldloc.1     // CS$0$0000
IL_001F:  ldtoken     LINQPad.User.TblEmployees.get_Age
IL_0024:  call        System.Reflection.MethodBase.GetMethodFromHandle
IL_0029:  castclass   System.Reflection.MethodInfo
IL_002E:  call        System.Linq.Expressions.Expression.Property
IL_0033:  ldc.i4.s    1E 
IL_0035:  box         System.Int32
IL_003A:  ldtoken     System.Int32
IL_003F:  call        System.Type.GetTypeFromHandle
IL_0044:  call        System.Linq.Expressions.Expression.Constant
IL_0049:  ldtoken     System.Nullable<System.Int32>
IL_004E:  call        System.Type.GetTypeFromHandle
IL_0053:  call        System.Linq.Expressions.Expression.Convert
IL_0058:  call        System.Linq.Expressions.Expression.GreaterThan
IL_005D:  ldc.i4.1    
IL_005E:  newarr      System.Linq.Expressions.ParameterExpression
IL_0063:  stloc.2     // CS$0$0001
IL_0064:  ldloc.2     // CS$0$0001
IL_0065:  ldc.i4.0    
IL_0066:  ldloc.1     // CS$0$0000
IL_0067:  stelem.ref  
IL_0068:  ldloc.2     // CS$0$0001
IL_0069:  call        System.Linq.Expressions.Expression.Lambda
IL_006E:  call        System.Linq.Queryable.Where
IL_0073:  call        System.Linq.Queryable.Count

00:00:00.0019229
00:00:00.0007023

-- Region Parameters
DECLARE @p0 Int = 1
-- EndRegion
SELECT COUNT(*) AS [value]
FROM [Employees] AS [t0]
WHERE [t0].[Type] = @p0

在我的测试中，所有员工都是空的。我得到这个结果：

var st=new Stopwatch();
st.Start();
db.TblEmployees.Where (t =>t.Age>30).Count ();
st.Stop();
Console.WriteLine(st.Elapsed);
st.Restart();
db.TblEmployees.Count (t =>t.Age>30);
st.Stop();
Console.WriteLine(st.Elapsed);

---

在性能上没有差别。这只是关于代码的可读性

我创建了一个表并在中运行这两个查询。输出结果如下：

方法1的SQL:

using (DBEntities db = new DBEntities())
{
   var employeeAgedAbove30 = db.Employees.Where(s => s.Age > 30).Count(); // Method 1

   employeeAgedAbove30 = db.Employees.Count(s => s.Age > 30); // Method 2

}

-- Region Parameters
DECLARE @p0 Int = 30
-- EndRegion
SELECT COUNT(*) AS [value]
FROM [tblEmployees] AS [t0]
WHERE [t0].[Age] > @p0
GO

-- Region Parameters
DECLARE @p0 Int = 30
-- EndRegion
SELECT COUNT(*) AS [value]
FROM [tblEmployees] AS [t0]
WHERE [t0].[Age] > @p0

IL_0001:  ldarg.0     
IL_0002:  stloc.0     // db
IL_0003:  ldloc.0     // db
IL_0004:  callvirt    LINQPad.User.TypedDataContext.get_TblEmployees
IL_0009:  ldtoken     LINQPad.User.TblEmployees
IL_000E:  call        System.Type.GetTypeFromHandle
IL_0013:  ldstr       "t"
IL_0018:  call        System.Linq.Expressions.Expression.Parameter
IL_001D:  stloc.1     // CS$0$0000
IL_001E:  ldloc.1     // CS$0$0000
IL_001F:  ldtoken     LINQPad.User.TblEmployees.get_Age
IL_0024:  call        System.Reflection.MethodBase.GetMethodFromHandle
IL_0029:  castclass   System.Reflection.MethodInfo
IL_002E:  call        System.Linq.Expressions.Expression.Property
IL_0033:  ldc.i4.s    1E 
IL_0035:  box         System.Int32
IL_003A:  ldtoken     System.Int32
IL_003F:  call        System.Type.GetTypeFromHandle
IL_0044:  call        System.Linq.Expressions.Expression.Constant
IL_0049:  ldtoken     System.Nullable<System.Int32>
IL_004E:  call        System.Type.GetTypeFromHandle
IL_0053:  call        System.Linq.Expressions.Expression.Convert
IL_0058:  call        System.Linq.Expressions.Expression.GreaterThan
IL_005D:  ldc.i4.1    
IL_005E:  newarr      System.Linq.Expressions.ParameterExpression
IL_0063:  stloc.2     // CS$0$0001
IL_0064:  ldloc.2     // CS$0$0001
IL_0065:  ldc.i4.0    
IL_0066:  ldloc.1     // CS$0$0000
IL_0067:  stelem.ref  
IL_0068:  ldloc.2     // CS$0$0001
IL_0069:  call        System.Linq.Expressions.Expression.Lambda
IL_006E:  call        System.Linq.Queryable.Where
IL_0073:  call        System.Linq.Queryable.Count

00:00:00.0019229
00:00:00.0007023

-- Region Parameters
DECLARE @p0 Int = 1
-- EndRegion
SELECT COUNT(*) AS [value]
FROM [Employees] AS [t0]
WHERE [t0].[Type] = @p0

执行时间：00:00.279

方法2的SQL:

using (DBEntities db = new DBEntities())
{
   var employeeAgedAbove30 = db.Employees.Where(s => s.Age > 30).Count(); // Method 1

   employeeAgedAbove30 = db.Employees.Count(s => s.Age > 30); // Method 2

}

-- Region Parameters
DECLARE @p0 Int = 30
-- EndRegion
SELECT COUNT(*) AS [value]
FROM [tblEmployees] AS [t0]
WHERE [t0].[Age] > @p0
GO

-- Region Parameters
DECLARE @p0 Int = 30
-- EndRegion
SELECT COUNT(*) AS [value]
FROM [tblEmployees] AS [t0]
WHERE [t0].[Age] > @p0

IL_0001:  ldarg.0     
IL_0002:  stloc.0     // db
IL_0003:  ldloc.0     // db
IL_0004:  callvirt    LINQPad.User.TypedDataContext.get_TblEmployees
IL_0009:  ldtoken     LINQPad.User.TblEmployees
IL_000E:  call        System.Type.GetTypeFromHandle
IL_0013:  ldstr       "t"
IL_0018:  call        System.Linq.Expressions.Expression.Parameter
IL_001D:  stloc.1     // CS$0$0000
IL_001E:  ldloc.1     // CS$0$0000
IL_001F:  ldtoken     LINQPad.User.TblEmployees.get_Age
IL_0024:  call        System.Reflection.MethodBase.GetMethodFromHandle
IL_0029:  castclass   System.Reflection.MethodInfo
IL_002E:  call        System.Linq.Expressions.Expression.Property
IL_0033:  ldc.i4.s    1E 
IL_0035:  box         System.Int32
IL_003A:  ldtoken     System.Int32
IL_003F:  call        System.Type.GetTypeFromHandle
IL_0044:  call        System.Linq.Expressions.Expression.Constant
IL_0049:  ldtoken     System.Nullable<System.Int32>
IL_004E:  call        System.Type.GetTypeFromHandle
IL_0053:  call        System.Linq.Expressions.Expression.Convert
IL_0058:  call        System.Linq.Expressions.Expression.GreaterThan
IL_005D:  ldc.i4.1    
IL_005E:  newarr      System.Linq.Expressions.ParameterExpression
IL_0063:  stloc.2     // CS$0$0001
IL_0064:  ldloc.2     // CS$0$0001
IL_0065:  ldc.i4.0    
IL_0066:  ldloc.1     // CS$0$0000
IL_0067:  stelem.ref  
IL_0068:  ldloc.2     // CS$0$0001
IL_0069:  call        System.Linq.Expressions.Expression.Lambda
IL_006E:  call        System.Linq.Queryable.Where
IL_0073:  call        System.Linq.Queryable.Count

00:00:00.0019229
00:00:00.0007023

-- Region Parameters
DECLARE @p0 Int = 1
-- EndRegion
SELECT COUNT(*) AS [value]
FROM [Employees] AS [t0]
WHERE [t0].[Type] = @p0

执行时间：00:00.275

---

在这里，您可以看到LINQ生成的SQL没有差别，执行时间也有细微差别。

第一个将首先过滤记录，然后计数，第二个将执行相同的操作。我认为，在itRun SQL Profiler（假设您使用MSSQL作为DB后端）或类似的操作中，第二个where子句位于itRun SQL Profiler中，捕获两个查询并比较它们。它们都被转换为相同的SQL（来自LINQPad）。@amarduplantier为什么不自己尝试一下呢。使用秒表测量时间：）顺便说一句，这两个语句都应该做同样的操作，了解详细信息。所以他们有

Where

只是为了可读性？我应该像其他人说的那样检查执行时间的差异吗？执行时间在我的项目中非常重要。@amarduplantier:更新了答案