How to add new column to a datatable?

You should be using Copy() instead of Clone(). Clone only copies the structure but copy method copies both structure and data.

So use

t1 = t2.Copy();

instead of

t1= t2.Clone();

<>>>>>>


t1=t2.copy()

you can modify your code like this

t1= t2.Clone();

t1.Columns.Add("Matching");

foreach(DataRow r in t1.Rows)
{
n = tags.Select("Title= '" + r["Title"] + "'");
    foreach (DataRow roo in n)
    {
         t1.ImportRow(roo);
        // match2["Matching"] = getscore();
    }
 foreach (DataRow roo in t1)
    {
         
        roo["Matching"] = getscore();
    }
}

or to add a column to a data table you can follow the below code

DataTable addNewColumn(DataTable t1)
    {
        // add a column to a table
        t1.Columns.Add("qty");

        int i = 1;
        foreach (DataRow dr1 in t1.Rows)
        {

            dr1["qty"] = i++;
        }

        return t1;

    }