实体框架 4：将字符串条件转换为 lambda 表达式？

发布于 2024-11-04 15:47:44 字数 421 浏览 2 评论 0原文

我想接受来自客户端的 where 条件的字符串数组，例如 field == value。如果创建一个规范对象，它可以接受构造函数中的字符串并输出 lambda 表达式来表示Where 子句，那就太好了。例如，我可以执行以下操作：

var myCondition = new Specification<Product>( myStringArrayOfConditions); 
var myProducts = DB.Products.Where( myCondition);

您如何将 "name == Jujyfruits" 转换为
DB.Products.Where(p => p.name == "JujyFruits “）？

原文

I want to accept a string array of where conditions from the client like field == value.
It would be really nice to create a specification object that could accept the string in the constructor and output a lambda expression to represent the Where clause. For example, I could do the following:

var myCondition = new Specification<Product>( myStringArrayOfConditions); 
var myProducts = DB.Products.Where( myCondition);

How could you turn "name == Jujyfruits" into
DB.Products.Where(p => p.name == "JujyFruits")?

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我很坚强 2024-11-11 15:47:44

您可以使用

Reflection 从字符串 name 中获取属性 Product.name，并使用
LINQ Expression 类来手动创建 lambda 表达式。

请注意，以下代码示例仅适用于Equals (==) 运算。然而，它也很容易推广到其他操作（按空格分割、解析运算符并选择适当的表达式而不是Expression.Equal）。

    var condition = "name == Jujyfruits";

    // Parse the condition
    var c = condition.Split(new string[] { "==" }, StringSplitOptions.None);
    var propertyName = c[0].Trim();
    var value = c[1].Trim();

    // Create the lambda
    var arg = Expression.Parameter(typeof(Product), "p");
    var property = typeof(Product).GetProperty(propertyName);
    var comparison = Expression.Equal(
        Expression.MakeMemberAccess(arg, property),
        Expression.Constant(value));
    var lambda = Expression.Lambda<Func<Product, bool>>(comparison, arg).Compile();

    // Test
    var prod1 = new Product() { name = "Test" };
    var prod2 = new Product() { name = "Jujyfruits" };
    Console.WriteLine(lambda(prod1));  // outputs False
    Console.WriteLine(lambda(prod2));  // outputs True

关于构造函数：由于 Func 是密封的，因此您无法从中派生。但是，您可以创建一个隐式转换运算符来转换Specification< ;T> 转换为 Func。

You can use

Reflection to get the Property Product.name from the string name and
the LINQ Expression class to manually create a lambda expression.

Note that the following code example will only work for Equals (==) operations. However, it is easy to generalize to other operations as well (split on whitespace, parse the operator and choose the appropriate Expression instead of Expression.Equal).

    var condition = "name == Jujyfruits";

    // Parse the condition
    var c = condition.Split(new string[] { "==" }, StringSplitOptions.None);
    var propertyName = c[0].Trim();
    var value = c[1].Trim();

    // Create the lambda
    var arg = Expression.Parameter(typeof(Product), "p");
    var property = typeof(Product).GetProperty(propertyName);
    var comparison = Expression.Equal(
        Expression.MakeMemberAccess(arg, property),
        Expression.Constant(value));
    var lambda = Expression.Lambda<Func<Product, bool>>(comparison, arg).Compile();

    // Test
    var prod1 = new Product() { name = "Test" };
    var prod2 = new Product() { name = "Jujyfruits" };
    Console.WriteLine(lambda(prod1));  // outputs False
    Console.WriteLine(lambda(prod2));  // outputs True

About the constructor thing: Since Func<T, TResult> is sealed, you cannot derive from it. However, you could create an implicit conversion operator that translates Specification<T> into Func<T, bool>.

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梦断已成空 2024-11-11 15:47:44

我们最近从 VS2008 示例项目中发现了动态 LINQ 库。非常适合将基于字符串的“Where”子句转换为表达式。

此链接将带您到达那里。

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居里长安 2024-11-11 15:47:44

您需要将搜索词变成谓词。尝试如下操作：

string searchString = "JujyFruits";
Func<Product, bool> search = new Func<Product,bool>(p => p.name == searchString);

return DB.Products.Where(search);

You need to turn your search term into a predicate. Try something like the following:

string searchString = "JujyFruits";
Func<Product, bool> search = new Func<Product,bool>(p => p.name == searchString);

return DB.Products.Where(search);

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