How to check if slice interface elements have the same dynamic type?

I have the following structs and they follow this structure:

A is the interface, and B, C, D are all types with interface A.

I have a slice of variables args all with type with interface A, each of which can be B, C, D type specifically.

I'd like to write a for loop to judge if all variables in the slice fall into the same dynamic type.

I wrote the following code:

var existingTyp A
for i, d := range args {
 switch typ := d.(type) {
   case *B, *C, *D:
    if existingTyp == nil {
        existingTyp = typ
    } else {
        if typ != existingTyp {
            panic("error!")
        }
   }
}

How to revise the code to achieve what I want?