根据列表属性删除重复对象

Question

我想仅根据某些属性来区分对象列表。这些属性是通过反射和某些条件获得的。我搜索了很多，但找不到任何能够在此 lambda 表达式中执行循环的代码片段或解决方案。

List<PropertyInfo> propList = ... 
var distinctList = FullList
  .GroupBy(uniqueObj => 
  { 
  //do a loop to iterate all elements in propList 
  })
  .Select(x => x.First());

Answer 1

好吧，我花了一段时间来考虑这个问题。

基本上，您可以使用 Linq GroupBy 运算符，但您需要使用接受自定义IEQualityComparer的重载，因为您希望根据对象所有属性的子集来验证对象的相等性。

属性的子集存储在代码中其他位置创建的List<PropertyInfo>中，或者从服务或其他位置接收的中。

因此，实现IEqualityComparer，然后将其与GroupBy一起使用：

//Dummy class representing your data.
//
//Notice that I made the IEqualityComparer as a child class only
//for the sake of demonstration
public class DataObject 
{
    public string Name { get; set; }
    public int Age { get; set; }
    public int Grade { get; set; }
    public static List<PropertyInfo> GetProps()
    {
        //Only return a subset of the DataObject class properties, simulating your List<PropertyInfo>
        return typeof(DataObject).GetProperties().Where(p => p.Name == "Name" || p.Name == "Grade").ToList();
    }

    public class DataObjectComparer : IEqualityComparer<DataObject>
    {
        public bool Equals(DataObject x, DataObject y)
        {
            if (x == null || y == null)
                return false;
            foreach (PropertyInfo pi in DataObject.GetProps())
            {
                if (!pi.GetValue(x).Equals(pi.GetValue(y)))
                    return false;
            }
            return true;
        }
        public int GetHashCode(DataObject obj)
        {
            int hash = 17;
            foreach (PropertyInfo pi in DataObject.GetProps())
            {
                hash = hash * 31 + pi.GetValue(obj).GetHashCode();
            }
            return hash;
        }
    }
}

//Then use that in your code:
//
List<DataObject> lst = new List<DataObject>();
lst.Add(new DataObject { Name = "Luc", Age = 49, Grade = 100 });
lst.Add(new DataObject { Name = "Luc", Age = 23, Grade = 100 });
lst.Add(new DataObject { Name = "Dan", Age = 49, Grade = 100 });
lst.Add(new DataObject { Name = "Dan", Age = 23, Grade = 100 });
lst.Add(new DataObject { Name = "Luc", Age = 20, Grade = 80 });
List<DataObject> dist = lst.GroupBy(p => p, new DataObject.DataObjectComparer()).Select(g => g.First()).ToList();    
//The resulting list now contains distinct objects based on the `Name` and `Grade` properties only.

我希望这可以帮助您更接近您的解决方案。

干杯

Answer 2

您可以通过此方法使用属性名称创建表达式：

public static Expression<Func<T, object>> GetPropertySelector<T>(string propertyName)
{
    var arg = Expression.Parameter(typeof(T), "x");
    var property = Expression.Property(arg, propertyName);
    //return the property as object
    var conv = Expression.Convert(property, typeof(object));
    var exp = Expression.Lambda<Func<T, object>>(conv, new ParameterExpression[] { arg });
    return exp;
}

并像这样使用：

var exp = GetPropertySelector<Person>("PropertyName");

现在，您可以轻松区分：

List<Person> distinctPeople = allPeople
  .GroupBy(exp.Compile())
  .Select(g => g.First())
  .ToList();