如何在LINQ中过滤具有不同参数的列表 [英] How to filter a list with varying parameters in LINQ

问题描述

假设我有一个这样的课程:

Suppose I have a class like this:

public class SampleClass
{
    public string SampleProperty1 { get; set; }
    public string SampleProperty2 { get; set; }
    public string SampleProperty3 { get; set; }
    public string SampleProperty4 { get; set; }
    public string SampleProperty5 { get; set; }
}

我有一个这样的列表:

List<SampleClass> sampleList = new List<SampleClass>();

我想按SampleProperty1和SampleProperty5过滤该列表.下次，我将使用SampleProperty3或SampleProperty2.我的意思是用户可以按自己想要的任何属性进行过滤.

I want to filter that list by SampleProperty1 and SampleProperty5. Next time I will use SampleProperty3 or SampleProperty2. I mean user can filter by any property he wants.

如何实现这种灵活性?

我不想写if语句和属性一样多，因为属性的实际数量更多.

I don't want to write if statements as many as properties because the real number of properties is a lot more.

有一种聪明的方法吗?

谢谢.

推荐答案

您可以为每个属性构建lambda表达式，并使用它过滤集合. 假设您有Dictionary个属性名称，以及要用于过滤的属性值:

You can build lambda expression for each property and filter the collection with it. Let's say you have Dictionary of property names with their values you want to filter by:

var filters = new Dictiontionary<string, object>();
IEnumerable<SampleClass> query = listOfSampleClasses;

// we will loop through the filters
foreach(filter in filters)
{
    // find the property of a given name
    var property = typeof(SampleClass).GetProperty(filter.Key, BindingFlags.Instance | BindingFlags.Public);
    if (property == null) continue;

    // create the ParameterExpression
    var parameter = Expression.Parameter(typeof(SampleClass));
    // and use that expression to get the expression of a property
    // like: x.SampleProperty1
    var memberExpression = Expression.Property(parameter, property);

    // Convert object type to the actual type of the property
    var value = Convert.ChangeType(filter.Value, property.PropertyType, CultureInfo.InvariantCulture);

    // Construct equal expression that compares MemberExpression for the property with converted value
    var eq = Expression.Equal(memberExpression, Expression.Constant(value));

    // Build lambda expresssion (x => x.SampleProperty == some-value)
    var lambdaExpression = Expression.Lambda<Func<SampleClass, bool>>(eq, parameter);

    // And finally use the expression to filter the collection
    query = query.Where(lambdaExpression);
}

var filteredList = query.ToList();

当然，您可以将该代码放入通用方法中，并过滤任何类型的集合.

Of course, you can put that code inside a generic method and filter collection of any type.

对于输入字典，包含两对:"SampleProperty1" - "foo"和"SampleProperty2" - "bar" 它将产生类似这样的内容:

For input dictionary, containing two pairs: "SampleProperty1" - "foo" and "SampleProperty2" - "bar" It will produce something like that:

listOfSampleClasses
    .Where(x => x.SampleProperty1 == "foo")
    .Where(x => x.SampleProperty2 == "bar");

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