string json = @"{
    ""name"": ""John"",
    ""age"": 30,
    ""city"": ""New York""
}";

DataTable dataTable = new DataTable();
dataTable.Columns.Add("Key", typeof(string));
dataTable.Columns.Add("Value", typeof(string));

foreach (var pair in json.Trim('{', '}').Split(','))
{
    var parts = pair.Split(':');
    dataTable.Rows.Add(parts[0].Trim('"'), parts[1].Trim('"'));
}