No best type found for implicitly-typed array

It's because you have two different anonymous types in the first example, the definition of the last item is different than the other ones.

In the first example , one containing an A property and one containing a B property, and the compiler can't figure out the type of array. In the second example there is one anonymous type, containing only A.

I think it's a typo, so you can change B to A in last entry in first example

From MSDN:

You can create an implicitly-typed array in which the type of the array instance is inferred from the elements specified in the array initializer.

You can use:

var marketValueData = new object[] {
    new { A = "" },
    new { A = "" },
    new { B = "" },
    ...,
};