НС>ints.Where(x => MyMethod(x, c1, c2) || (x > c3 && x < c4));
НС>[ExpressionMethod("MyMethodExpr")]
НС>private static bool MyMethod(int x, int c1, int c2)
НС>{
НС>    return (x > c1 && x < c2);
НС>}
НС>private static Expression<Func<int, int, int, bool>> MyMethodExpr()
НС>{
НС>    return (x, c1, c2) => x > c1 && x < c2;
НС>}
НС>

IEnumerable<int> ints = ...;
ints.Where(x => MyMethod(x, c1, c2, myEnum) || (x > c3 && x < c4));

private static bool MyMethod(int x, int c1, int c2, MyEnum myEnum)
{
    switch (myEnum)
    {
        case MyEnum.Item1:
            return x > c1 && x < c2;
        case MyEnum.Item2:
            return x > c1 && x < c2*c1;
        case MyEnum.Item3:
            return x > c1 && x < c2%c1*17;
        default:
            throw new ArgumentOutOfRangeException("myEnum");
    }
}