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This article mainly explains how to use linq dynamic condition query. Interested friends may wish to have a look at it. The method introduced in this paper is simple, fast and practical. Let's let the editor take you to learn how to use linq dynamic condition query.

1Construction of expression tree for dynamic conditions of LINQ

Private Expression getCondition ()

{

Expression expression = blog = > true

If (! String.IsNullOrEmpty (Request ["BlogClassID"]))

{

Int blogClassID

If (Int32.TryParse (Request ["BlogClassID"], out blogClassID))

{

Expression e2 = blog = >

Blog.BlogClass = = null

Var invokedExpr = Expression.Invoke

(e, expression.Parameters.Cast

())

Expression = Expression.Lambda

(Expression.And (expression.Body, invokedExpr), expression.Parameters)

}

}

Return expression

}

The main query looks like this:

Var result = new DongBlogDataContext () .Blogs.Where (getCondition ()

Because according to SQL tracking, generating SQL is similar:

SELECT [t0]. [BlogID], [t0]. [ChannelID], [t0]. [BlogClassID], [t0]. [Title], [t0]. [Content], [t0]. [Tag], [t0]. [CreateDateTime] FROM [dbo]. [Blog] AS [t0] WHERE [t0]. [BlogClassID] IS NULL

The essence of this method is to merge Lamba expressions, that is, these three sentences.

SELECT [t0]. [BlogID], [t0]. [ChannelID], [t0]. [BlogClassID], [t0]. [Title], [t0]. [Content], [t0]. [Tag], [t0]. [CreateDateTime] FROM [dbo]. [Blog] AS [t0] WHERE [t0]. [BlogClassID] IS NULL

It would be troublesome to write this for every conditional merge, but fortunately someone has already written a helper class:

Using System; using System.Linq; using System.Linq.Expressions; using System.Collections.Generic; public static class PredicateBuilder {public static Expression True () {return f = > true;} public static Expression False () {return f = > false;} public static Expression Or (this Expression expr1, Expression expr2) {var invokedExpr = Expression.Invoke (expr2, expr1.Parameters.Cast ()) Return Expression.Lambda (Expression.Or (expr1.Body, invokedExpr), expr1.Parameters);} public static Expression And (this Expression expr1, Expression expr2) {var invokedExpr = Expression.Invoke (expr2, expr1.Parameters.Cast ()); return Expression.Lambda (Expression.And (expr1.Body, invokedExpr), expr1.Parameters) }}

This class can be used for merging expressions of type Expression.

2 the construction of Query for the dynamic condition of LINQ

It is better to write the same query:

Private IQueryable getQuery () {IQueryable query = new DongBlogDataContext (). Blogs; if (! String.IsNullOrEmpty (Request ["BlogClassID"])) {int blogClassID; if (Int32.TryParse (Request ["BlogClassID"], out blogClassID)) query = query.Where (blog = > blog.BlogClass = = null);} return query.OrderByDescending (blog = > blog.CreateDateTime) }

Main query

Var result = getQuery ()

The generated SQL and * * are exactly the same.

At this point, I believe you have a deeper understanding of "how to use linq dynamic condition query". You might as well do it in practice. Here is the website, more related content can enter the relevant channels to inquire, follow us, continue to learn!

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