gistfile1.cs

private void Hoge(string kubun)
{
    var dataList = new[] {
        new { Value1 = "aaa", Value2 = (string)null },
        new { Value1 = "bbb", Value2 = (string)null },
        new { Value1 = (string)null, Value2 = "aaa" },
        new { Value1 = (string)null, Value2 = "bbb" },
    };

    var data1 = dataList.First(n => kubun == "1" ? n.Value1 == "aaa" : n.Value2 == "bbb");

    //Compile Error.
    //var data2 = dataList.First(kubun == "1" ? n => n.Value1 == "aaa" : n => n.Value2 == "bbb");

    var data3 = (kubun == "1") 
        ? dataList.First(n => n.Value1 == "aaa")
        : dataList.First(n => n.Value2 == "bbb");
}