使用接口列表时，如何在C＃中深度克隆列表？

Question

I have the following interface

public interface IReportFilter
{
    IReportColumn ReportColumn { get; set; }

    FilterType Type { get; set; }

    string Value { get; set; }

    string FormattedValue { get; }

    string BuildSqlFilter(string parameterName);

    List<IReportFilter> SubFilters { get; set; }

    FilterOperator SqlOperator { get; set; }

    FilterOperator SubFiltersOperator { get; set; }
}

I have a class that implements it like this

public class ReportFilter : IReportFilter
{
    ...
    ...
}

From another class in my project I have the following code

List<IReportFilter> filters = new List<IReportFilter>
{
    new ReportFilter
    { 
        ReportColumn = new ReportColumn{ ColumnKey = "Result.IsCompleted"},
        Value = "1",
        SubFilters = new List<IReportFilter> 
        {
             new ReportFilter { SqlOperator = FilterOperator.Or, ReportColumn = new ReportColumn{ ColumnKey = "User.LastName"}, Value = "Jones"},
             new ReportFilter { SqlOperator = FilterOperator.Or, ReportColumn = new ReportColumn{ ColumnKey = "User.LastName"}, Value = "Smith"},
             new ReportFilter { SqlOperator = FilterOperator.Or, ReportColumn = new ReportColumn{ AggregateFunction = SqlAggregateFunctions.Count}, Type = FilterType.GreaterThenOrEqualTo ,Value = "0" },
        }
    },
};

I like to be able to pass my filters object to a method which will separate my filter based on a condition.

Here is a method that I write to separate my filters recursively

private List<IReportFilter> ExtractFiltersByAType(List<IReportFilter> filters, bool IsStandard = true)
{
    List<IReportFilter> validFilters = new List<IReportFilter>();

    foreach(var filter in filters)
    {

        if (filter.SubFilters != null && filter.SubFilters.Any())
        {
            //At this point we know there are some sub filters
            filter.SubFilters = ExtractFiltersByAType(filter.SubFilters, IsStandard);
        }

        if( (IsStandard && !IsAggregate(filter.ReportColumn.AggregateFunction) ) || (!IsStandard && IsAggregate(filter.ReportColumn.AggregateFunction) ) )
        {
            validFilters.Add(filter);
        }

    }

    return validFilters;
}

The problem with this method is that the variable filters gets passed by reference instead of creating a copy an object. Which means any change I make will change the original filters object! I really don't understand this behavior in C# and is making my life miserable!

Anyhow, I did some research and I found out that I need to deep clone my filters object before passing it to my method.

I tried to use the following method to deep clone my object

public static T Clone<T>(T source)
{
    var serialized = JsonConvert.SerializeObject(source);
    return JsonConvert.DeserializeObject<T>(serialized);
}

Then I used it like so

var copyOfFilters = Clone(filters);

but this is giving me an error because I my SubFilters object is constructed with an interface!

Could not create an instance of type ReportsEngine.Support.ReportsGenerator.Report.Contracts.IReportFilter. Type is an interface or abstract class and cannot be instantiated. Path '[0].ReportColumn', line 1, position 17.

How can I correctly clone my object?

I am also curios to know which why such behavior exists in c#?

Answer 1

I am also curios to know which why such behavior exists in c#?

You cannot instantiate interfaces. Interfaces only give a contract that types needs to adhere to. It does not give an implementation for that contract though.

So when you attempt to deserialize some JSON into an interface type, this does not work since the deserializer cannot instantiate that (interface) type. It needs a concrete type instead—for example your ReportFilter . So you could call JsonConvert.DeserializeObject<ReportFilter>(serialized) and it would work.

However, since you are doing this in a generic method, you attempt to deserialize it as type T . That type T comes from the generic type argument of the method which is inferred from the call to that method. In this case, you are likely passing a IReportFilter object, so at compile time that's the type that's being used for T (regardless of whether that object is an actual ReportFilter ).

In order to change that, you would have to call it explicitly as a ReportFilter :

var copyOfFilter = Clone((ReportFilter)filter);