Передайте FieldName в качестве параметра

internal class Program {
    private static void Main(string[] args) {
        var bar = new Bar {A = 1, B = 2, C = 3};
        Console.WriteLine(new Foo().X(bar, it => it.A));
        Console.WriteLine(new Foo().X(bar, it => it.B));

        // or introduce a "constant" somewhere
        Func cFieldSelector = it => it.C;
        Console.WriteLine(new Foo().X(bar, cFieldSelector));
    }
}

internal class Foo {
    // Instead of using a field by name, give it a method to select field you want.
    public int X(Bar bar, Func fieldSelector) {
        return fieldSelector(bar) * 2;
    }

    public int A(Bar bar) {
        return bar.A * 2;
    }

    public int B(Bar bar) {
        return bar.B * 2;
    }

    public int C(Bar bar) {
        return bar.C * 2;
    }
}

internal class Bar {
    public int A;
    public int B;
    public int C;
}
.cs-file