ifthenelse{equal{x}{y}}{XXX}{YYY}

newcommand{tmpcmd}[1]
{
    textsf{#1}
    ifthenelse{equal{x}{y}}{XXX}{YYY}
}

tmpcmd{C}

! Undefined control sequence.
<argument> equal
                  {x}{y}

newcommand{pn}[1]
{
    textsf{#1}
}

newcommand{pn}[1]
{
    textsf{#1}
    ifthenelse{equal{x}{y}}{XXX}{YYY}
}