Compare and match two DataTable and insert in another DataTable using C# and VB.Net

Refer the below sample code and implement it in your code as per your code logic.

C#

DataTable dt = new DataTable();
dt.Columns.Add(new DataColumn("Id", typeof(string)));
dt.Rows.Add("test1@test.com,test2@test.com,test3@test.com");
dt.Rows.Add("test4@test.com,test5@test.com,test6@test.com");
dt.Rows.Add("test7@test.com,test8@test.com,test9@test.com");
DataTable dtusercheck = dt;

DataTable dt2 = new DataTable();
dt2.Columns.Add(new DataColumn("Id", typeof(string)));
dt2.Rows.Add("test3@test.com");
DataTable dtuserinprojectornot = dt2;
DataTable dt3 = dtuserinprojectornot.Clone();

foreach (DataRow row1 in dtusercheck.Rows)
{
    foreach (DataRow row2 in dtuserinprojectornot.Rows)
    {
        foreach (string emailId in row1["id"].ToString().Split(','))
        {
            if (emailId == row2["id"].ToString())
            {
                dt3.ImportRow(row2);
            }
        }
    }
}
if (dt3.Rows.Count > 0)
{
     lblErrMsg.Visible = true;
}
else
{
    lblErrMsg.Visible = true;
}

VB.Net

Dim dt As DataTable = New DataTable()
dt.Columns.Add(New DataColumn("Id", GetType(String)))
dt.Rows.Add("test1@test.com,test2@test.com,test3@test.com")
dt.Rows.Add("test4@test.com,test5@test.com,test6@test.com")
dt.Rows.Add("test7@test.com,test8@test.com,test9@test.com")
Dim dtusercheck As DataTable = dt
Dim dt2 As DataTable = New DataTable()
dt2.Columns.Add(New DataColumn("Id", GetType(String)))
dt2.Rows.Add("test3@test.com")
Dim dtuserinprojectornot As DataTable = dt2
Dim dt3 As DataTable = dtuserinprojectornot.Clone()

For Each row1 As DataRow In dtusercheck.Rows
    For Each row2 As DataRow In dtuserinprojectornot.Rows
        For Each emailId As String In row1("id").ToString().Split(","c)
            If emailId = row2("id").ToString() Then
                dt3.ImportRow(row2)
            End If
        Next
    Next
Next

If dt3.Rows.Count > 0 Then
    lblErrMsg.Visible = True
Else
    lblErrMsg.Visible = True
End If