Question

我想构建动态查询但未能获得所需的结果。请参阅以下代码。我尝试过使用两种不同的方法。

简单的Linq，结果是以下代码中的“resultUsingLing”。
动态Linq结果是“resultUsingDynamicLinq”，如下所示代码。

我想从“resultUsingLing”和“resultUsingDynamicLinq”获得相同的结果。 resultUsingDynamicLinq不返回任何值。

public class Test
{
    public string Name { get; set; }
    public string Address { get; set; }
}
public class Program
{
    public static void Main()
    {
        List<Test> testList = new List<Test>() 
        {
            new Test() { Address = "Address", Name = "Name" },
            new Test() { Address = "Address1", Name = "Name1" }, 
            new Test() { Address = "Address2", Name = "Name2" }, 
            new Test() { Address = "Address3", Name = "Name3" },
            new Test() { Address = "Address4", Name = "Name4" } 
        };
        IQueryable<Test> queryableTestData = testList.AsQueryable<Test>();
        var resultUsingLing= queryableTestData.Where(x => x.Address.Equals("Address1"));

        string property = "Address";
        string value = "Address1";
        var type = typeof(Test);
        var pe1 = Expression.Parameter(type, "p");
        var propertyReference = Expression.Property(pe1, property);
        var constantReference = Expression.Constant(value);
        var resultUsingDynamicLinq= Expression.Lambda<Func<Test, bool>>
            (Expression.Equal(propertyReference, constantReference),
            new[] { pe1 }).Compile();
    }
}

Answer 1

那是因为你没有使用你在任何地方创建过的表达。

顺便说一下，你不应该编译它。编译为您提供Func<Test, bool>，这将使您的Where来电使用IEnumerable.Where，而非IQueryable.Where。

var expression = Expression.Lambda<Func<Test, bool>>
    (Expression.Equal(propertyReference, constantReference),
    new[] { pe1 });

var resultUsingDynamicLinq = queryableTestData.Where(expression).ToList();

动态Linq查询和表达

1 个答案: