How to call a Generic method on a dynamic type in C#

Question

I have the following code:

public static List<object[]> Serialise2D_Rec<T>(IEnumerable<T> data)
{
    int numElts = 0;
    foreach (var item in data)
        numElts++;

    Type t = typeof(T); // Get type pointer
    PropertyInfo[] propList = t.GetProperties();


    List<object[]> toret = new List<object[]>();

    for (long propID = 0; propID < propList.Count(); ++propID)
    {
        var proptype = propList[propID].PropertyType;
        if (proptype.IsPrimitive || proptype == typeof(Decimal) || proptype == typeof(String))
        {
            toret.Add(new object[numElts + 1]);
            toret[toret.Count - 1][0] = propList[propID].Name;
            int row = 1;
            foreach (T item in data)
            {
                toret[toret.Count - 1][row] = propList[propID].GetValue(item, null);
                row++;
            }
        }
        else
        {
            var lst = (IList)Activator.CreateInstance((typeof(List<>).MakeGenericType(proptype)));
            foreach (T item in data)
            {
                lst.Add(propList[propID].GetValue(item, null));
            }
            List<object[]> serialisedProp = Serialise2D_Rec(lst);

        }
    }
    return toret;

}

However this line will fail with:

List<object[]> serialisedProp = Serialise2D_Rec(lst);

With the error:

****: error CS0411: The type arguments for method '****.****.Serialise2D_Rec<T>(System.Collections.Generic.IEnumerable<T>)' cannot be inferred from the usage. Try specifying the type arguments explicitly.

How can I specify the type in the recursion, it seems that the syntax for dynamic generic types is not that trivial.

Answer 1

I don't see a valid use case for generics in your case. Usage of generics assume you are able to statically identify the type on compile-time (or its ancestor, at least). Why don't you just accept the non-generic IEnumerable? If you do need to supply some base type of items in data, then supply it as a parameter:

public static List<object[]> Serialise2D_Rec(IEnumerable data, Type t)
{
    …
    for (…)
    {
        if (…)
        {
        }
        else
        {
            …
            List<object[]> serialisedProp = Serialise2D_Rec(lst, proptype);
        }
    }
}

---

Side note: use the extension method data.Count() instead of foreach (var item in data) numElts++;.

Answer 2

Since your type is dynamic, you won't know the type of the generic parameter until run-time. For this reason, you must also treat the function as dynamic, because you won't know the "type" of the function's generic signature until run-time.

You must use reflection to call a generic function dynamically. See How do I use reflection to call a generic method?

Answer 3

Try this:

// Get The Method by reflection
    MethodInfo serializerInfo = typeof(Serializer).GetMethod("Serialise2D_Rec",
                        BindingFlags.Public | BindingFlags.Static); 

//Make a Generic instance of the type you want
    serializerInfo = serializerInfo.MakeGenericMethod(lst.GetType());

    List<object[]> serialisedProp = serializerInfo.Invoke(null, new object[] {lst});

Instead of Serializer in the typeof() put the class that hold your func Serialise2D_Rec

Answer 4

It doesn't seem like you can use generics in this case. The line fails because lst is a non-generic IList and it's passed as arguent to Serialise2D_Rec, which requires an IEnumerable<T>.

I suggest that you change your method not to be generic; it uses reflection anyway, it may not have a big impact.

Try this:

public static List<object[]> Serialise2D_Rec(IList data)
{
    int numElts = 0;
    foreach (var item in data)
        numElts++;

    if (data.Count == 0)
        throw new Exception("Cannot handle empty lists.");

    Type t = data[0].GetType(); // Get type pointer
    PropertyInfo[] propList = t.GetProperties();

    List<object[]> toret = new List<object[]>();

    for (long propID = 0; propID < propList.Count(); ++propID)
    {
        var proptype = propList[propID].PropertyType;
        if (proptype.IsPrimitive || proptype == typeof(Decimal) || proptype == typeof(String))
        {
            toret.Add(new object[numElts + 1]);
            toret[toret.Count - 1][0] = propList[propID].Name;
            int row = 1;
            foreach (object item in data)
            {
                toret[toret.Count - 1][row] = propList[propID].GetValue(item, null);
                row++;
            }
        }
        else
        {
            var lst = (IList)Activator.CreateInstance((typeof(List<>).MakeGenericType(proptype)));
            foreach (object item in data)
            {
                lst.Add(propList[propID].GetValue(item, null));
            }
            List<object[]> serialisedProp = Serialise2D_Rec(lst);

        }
    }

    return toret;
}

Answer 5

Yes, that's because lst is of type object.

You'll need to dynamically invoke the correct generic type of Serialise2D_Rec<T>()