C# Two dimensional string out parameter

I have a .Net component and a VB Client application which calls a method of the
component class

The parameter of the chart component is a out two dimensional string array
which will be filled with string values once the component function returns
successfully.

The issue is when it returns to VB Client application the elements in the string array
becomes null.

The prototype of the component function is like this

public void GetChartDetails ([MarshalAs( UnmanagedType.SafeArray,SafeArraySubType = VarEnum.VT_BSTR)]ref String []chartDetails )


and from the client VB application it is called like this

Dim tempString(3) As String
Dim TempObj As New T1TempXML.T1OATempXML

TempObj .GetChartDetails (tempString)

tempString array is getting filled with null values after calling the component function.