如果在 sql 到 linq 中存在语句
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原文地址: http://stackoverflow.com/questions/476214/
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if exists statement in sql to linq
提问by Subliminal Hash
What is the linq equivalent of the following statement ?
以下语句的 linq 等效项是什么？
IF NOT EXISTS(SELECT UserName FROM Users WHERE UserName='michael')
BEGIN
INSERT INTO Users (UserName) values ('michael');
END
also can you suggest any sql-to-linq converters? I am currently using LINQPad which does a great job in terms of writing linq code where you can also see the generated sql code however when I click the little linq sign, nothing is displayed.
你也可以推荐任何 sql-to-linq 转换器吗？我目前正在使用 LINQPad，它在编写 linq 代码方面做得很好，您还可以在其中看到生成的 sql 代码，但是当我单击小 linq 标志时，没有显示任何内容。
回答by tvanfosson
It can't be done in LINQ2SQL with a single statement as the LINQ syntax and extension methods don't support inserts. The following (assuming a datacontext named db) should do the trick.
由于 LINQ 语法和扩展方法不支持插入，因此无法在 LINQ2SQL 中使用单个语句完成。以下（假设名为 的数据上下文db）应该可以解决问题。
if (!db.Users.Any( u => u.UserName == "michael" ))
{
db.Users.InsertOnSubmit( new User { UserName = "michael" } );
db.SubmitChanges();
}
回答by Michael
Extension method that implements tvanfosson's solution:
实现tvanfosson解决方案的扩展方法：
/// <summary>
/// Method that provides the T-SQL EXISTS call for any IQueryable (thus extending Linq).
/// </summary>
/// <remarks>Returns whether or not the predicate conditions exists at least one time.</remarks>
public static bool Exists<TSource>(this IQueryable<TSource> source, Expression<Func<TSource, bool>> predicate)
{
return source.Where(predicate).Any();
}
/// <summary>
/// Method that provides the T-SQL EXISTS call for any IQueryable (thus extending Linq).
/// </summary>
/// <remarks>Returns whether or not the predicate conditions exists at least one time.</remarks>
public static bool Exists<TSource>(this IQueryable<TSource> source, Expression<Func<TSource, int, bool>> predicate)
{
return source.Where(predicate).Any();
}
The extension method would then be used:
然后将使用扩展方法：
bool exists = dataContext.Widgets.Exists(a => a.Name == "Premier Widget");
Although the .Where().Any() combination works sufficiently, it does certainly help the logic flow of the code presentation.
尽管 .Where().Any() 组合足够有效，但它确实有助于代码呈现的逻辑流程。
回答by Brian Wells
Put the Exists code in a static class. e.g. add a class to your project w/ something like:
将 Exists 代码放在静态类中。例如，将一个类添加到您的项目中，例如：
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Linq.Expressions;
namespace company.project
{
static class LinqExtensions
{
/// <summary>
/// Method that provides the T-SQL EXISTS call for any IQueryable (thus extending Linq).
/// </summary>
/// <remarks>Returns whether or not the predicate conditions exists at least one time.</remarks>
public static bool Exists<TSource>(this IQueryable<TSource> source, Expression<Func<TSource, bool>> predicate)
{
return source.Where(predicate).Any();
}
/// <summary>
/// Method that provides the T-SQL EXISTS call for any IQueryable (thus extending Linq).
/// </summary>
/// <remarks>Returns whether or not the predicate conditions exists at least one time.</remarks>
public static bool Exists<TSource>(this IQueryable<TSource> source, Expression<Func<TSource, int, bool>> predicate)
{
return source.Where(predicate).Any();
}
}
}
Don't forget to add the namespace of this class to any other class using it. ;P
不要忘记将此类的命名空间添加到使用它的任何其他类。;P
