if cell value is negative then set in bracket . how?

eg. If cell value is -45 then change the cell value to "(45)" like that.

I am doing like that.

UltragridRow.Cells[i].Value="(45)"

but my cell datatype is double thats why above statement gives error .

error is - unable to convert from system.double to system.string.

Can we change cell data type instead of changing datasourse column data type.