The difference between Expression<Func<T, bool>> and Func<T, bool> in EF

Little scum · Posted on 9/26/2021 9:21:21 AM

Func<TObject, bool> is a delegate

Expression<Func<TObject, bool>> are expressions

Expression will become a delegate after compilation before it can be run. Like what

Expression<Func<int, bool>> ex = x=>x < 100;

Func<int, bool> func = ex. Compile();

Then you can call func:

func(5) //-returns true

func(200) //- returns false

Expressions cannot be called directly.

Case:Incorrect query code causes a full table query of the database。

The error code is as follows:

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Pass a variable of type Func as an argument to the Count method,EF queries all the data in the entire table and filters it from memory。 The generated SQL statement looks like this:

info: Microsoft.EntityFrameworkCore.Database.Command[20101]
   Executed DbCommand (8ms) [Parameters=[], CommandType='Text', CommandTimeout='30']
   SELECT [c]. [id], [c]. [client_type], [c]. [status], [c]. [tenant]
   FROM [table] AS [c]

The correct code is as follows:

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The correct writing adds a where condition from the database and then returns the value of the count directly。 The generated SQL statement looks like this:

info: Microsoft.EntityFrameworkCore.Database.Command[20101]
   Executed DbCommand (4ms) [Parameters=[], CommandType='Text', CommandTimeout='30']
   SELECT COUNT(*)
   FROM [table] AS [c]
   WHERE [c]. [client_type] = 1

EF Core Series 4 looks at the generated SQL statements
https://www.itsvse.com/thread-9564-1-1.html

ef displays the generated SQL statements
https://www.itsvse.com/thread-3813-1-1.html

Using Func delegate calls areSystem.Linq.EnumerableCount below

The expression called with Expression<Func<T, bool>> isSystem.Linq.QueryableCount below

(End)

602447933 · Posted on 9/26/2021 4:19:52 PM

learned

flying fish · Posted on 9/26/2021 5:31:25 PM

Yes, I'm here to learn again...

[Source] The difference between Expression<Func<T, bool>> and Func<T, bool> in EF

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