My code:
return (Restitution >= 0 && Restitution <=1 && Hardness >= 0 && Hardness <= 1);

In reflector:

[return: MarshalAs(UnmanagedType.U1)]
private bool IsValid()
{
    int num;
    if (((this.Restitution >= 0f) && (this.Restitution <= 1f)) && ((this.Hardness >= 0f) && (this.Hardness <= 1f)))
    {
        num = 1;
    }
    else
    {
        num = 0;
    }
    return (bool) ((byte) num);