How to Correctly Use LINQ Join and Lambda Expressions to Avoid Common Query Errors?

LINQ and Lambda expression Join/where: solve the query error

When using Linq's Join and Lambda expressions, errors are often encountered. Let us explore a common wrong scene and explore clear solutions.

The following queries are designed to connect two tables based on the conditions of ID, but it will cause multiple errors:

In order to correct these errors, please consider the following options:

<code class="language-c#">int id = 1;
var query = database.Posts.Join(
    database.Post_Metas,
    post => database.Posts.Where(x => x.ID == id),
    meta => database.Post_Metas.Where(x => x.Post_ID == id),
    (post, meta) => new { Post = post, Meta = meta }
);</code>

<.> 1. Use Linq to query grammar:

In order to obtain a more natural and more similar to SQL, please select the linq query grammar:

<.> 2. Use linq expansion method:

<code class="language-c#">var id = 1;
var query =
   from post in database.Posts
   join meta in database.Post_Metas on post.ID equals meta.Post_ID
   where post.ID == id
   select new { Post = post, Meta = meta };</code>

If you insist on using Lambda expression, please adjust the grammar as follows:

Please keep in mind that when using Lambda expression:

<code class="language-c#">var id = 1;
var query = database.Posts    // 起始点（表等效项）
   .Join(database.Post_Metas, // 内连接表
      post => post.ID,        // 选择主键
      meta => meta.Post_ID,   // 选择外键
      (post, meta) => new { Post = post, Meta = meta }) // 选择
   .Where(postAndMeta => postAndMeta.Post.ID == id);    // 筛选条件</code>

The outer bracket contains Join operation.

The first two LAMBDA expressions are selected to connect the main and external keys.

• The third lambda expression defines the choice of result attributes.
• WHERE clause definition screening conditions.

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