Facing a issue error called "error C2228: left of '.substr' must have class/struct/union"

See more:

VisualC++

I am new to c++ ... i want to split a string into two equal parts . I am getting the build error

" C2228: left of '.substr' must have class/struct/union"

What I have tried:

void Divide(String^ messageText)
{
int length = sizeof(messageText);
int sublength = length/2;
String ^FirstString;
FirstString = messageText.substr (0,sublength);
}