如何过滤在LINQ有不同的参数列表

Question

假设我有一个这样的类：

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; } 
} 

而且我有一个这样的名单：

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

我要过滤通过名单SampleProperty1和SampleProperty5。下次我会用SampleProperty3或SampleProperty2。我的意思是用户可以过滤他想要的任何属性。

我该如何实现这种灵活性？

我不想编写if语句和属性一样多，因为属性的实际数量更多。

有没有一个聪明的方法来做到这一点？

谢谢。

Answer 1

您可以为每个属性构建lambda表达式并使用它过滤集合。 比方说，你有属性名称的Dictionary与要通过筛选自己的价值观：

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，你可以把这些代码的任何类型的一般方法和过滤器内部集合。

对于输入字典，包含两个对："SampleProperty1" - "foo"和"SampleProperty2" - "bar" 它会产生类似的东西：

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

Answer 2

你可以用这样的条件构建一个LINQ语句。

var query = sampleList; 

if(shouldFilterProperty1) 
{ 
    query = query.Where(x => x.SampleProperty1.Contains(SearchPattern1)); 
} 
if(shouldFilterProperty2) 
{ 
    query = query.Where(x => x.SampleProperty2.Contains(SearchPattern2)); 
} 

var result = query.ToList(); 

这只是前两个属性的一个示例。如果您需要其他检查（而不是Contains），则可以使用相同的样式来执行它们。

Answer 3

如果你有能够改变你SampleClass类的结构奢侈品，那么你可以尝试像以下：

public class SampleClass 
{ 
    public IDictionary<string, string> Properties { get; } 
} 

private static IList<SampleClass> FilterList(IList<SampleClass> list, 
              params Tuple<string, string>[] propertyNames) 
{ 
    // No point filtering here. 
    if (propertyNames == null) 
    { 
     return list; // or null/empty list if you want to match none as the default. 
    } 

    // Match All the property values supplied in the filter, you can change this to 
    // Contains or string.Equals, etc. to suit your matching needs. 
    return list.Where(x => propertyNames.All(p => x.Properties[p.Item1] == p.Item2)) 
       .ToList(); 
} 

用法：

FilterList(sampleList, 
      Tuple.Create("SampleProperty1", "Value1"), 
      Tuple.Create("SampleProperty5", "Value5")); 

这种方法的好处意味着您不需要大量的if语句就可以确定原来要求的Where子句。

Answer 4

你可以找到你想要通过反射筛选和使用这种方法来构建LINQ查询性能。

// create your sample list 
List<SampleClass> sampleList = new List<SampleClass>(); 
sampleList.Add(...) 

// create the filter 
Dictionary<string, string> Filter = new Dictionary<string, string>(); 
Filter.Add("SampleProperty1", "SearchPattern1"); 
Filter.Add("SampleProperty5", "SearchPattern5"); 

// create the linq query 
var query = sampleList.AsEnumerable();  
foreach(var filterItem in Filter) 
{ 
    // get the property you want to filter 
    var propertyInfo = typeof(SampleClass).GetProperty(filterItem.Key); 

    // add the filter to your query 
    query = query.Where(x => (string)propertyInfo.GetValue(x) == filterItem.Value); 
} 

// execute the query 
var resultList = query.ToList();