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.

@HåkanFahlstedt: It's coming from near the end as part of the recursion; it's compile-time type is IList (non-generic) — Chris Sinclair
– Chris Sinclair, Commented Jan 27, 2014 at 17:47
Ok, as you can see in the answers there is a problem with using a non-generic type here. — Håkan Fahlstedt
– Håkan Fahlstedt, Commented Jan 27, 2014 at 17:53

Ondrej Tucny · Accepted Answer · 2014-01-27 17:47:23Z

4

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++;.

answered Jan 27, 2014 at 17:47

Ondrej Tucny

28.2k6 gold badges73 silver badges93 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

BlueTrin Over a year ago

Duly noted, however I am still interesting in knowing how I can create the call to the method dynamically. BTW in this case I need T to get the headers correctly in the case the IEnumerable does not contain an element.

Ondrej Tucny Over a year ago

You'd have to retrieve MethodInfo for Serialize2D_Rec and the use MethodInfo.MakeGenericMethod to build an instance matching proptype. However, I doubt it's worth the clutter that will arise in your code.

Community · Accepted Answer · 2017-05-23 12:21:00Z

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?

edited May 23, 2017 at 12:21

CommunityBot

11 silver badge

answered Jan 27, 2014 at 17:48

JDB

26.1k5 gold badges81 silver badges132 bronze badges

3 Comments

BlueTrin Over a year ago

Thanks, I ended up using this one.

JDB Over a year ago

@BlueTrin - Make sure to read the second answer on that question. Might be worth using for the sake of code maintainability.

BlueTrin Over a year ago

This is really good stuff, I will use it when I get back to the office. In my case I know the most common types that will be called, so I could add this for the sake of having the check.

qwark · Accepted Answer · 2014-01-27 17:48:13Z

1

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

answered Jan 27, 2014 at 17:48

qwark

5431 gold badge4 silver badges16 bronze badges

Comments

Doug · Accepted Answer · 2014-01-27 17:49:07Z

1

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;
}

answered Jan 27, 2014 at 17:49

Doug

6,4603 gold badges36 silver badges53 bronze badges

1 Comment

BlueTrin Over a year ago

I voted up your answer, but in my case I need it to work even on empty IEnumerable<T>, because it prints the name of the property in the first element of the Object[]: it is used later because the user is an Excel addin, so we print the headers of the name of the properties of the object even if there was no item returned.

Jon G · Accepted Answer · 2014-01-27 17:47:29Z

0

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

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

answered Jan 27, 2014 at 17:47

Jon G

4,16424 silver badges28 bronze badges

1 Comment

BlueTrin Over a year ago

Well how do I build the correct type using Invoke and MakeGenericMethod ?

Collectives™ on Stack Overflow

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

5 Answers 5

2 Comments

3 Comments

Comments

1 Comment

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

5 Answers 5

2 Comments

3 Comments

Comments

1 Comment

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related