C#, Looping through dataset and show each record from a dataset column

Looping through the rows, updating the values as you go is definitely a solution, and a quite easy one:

foreach (DataRow row in table.Rows)
{
    if (row.IsNull("foo")) row["foo"] = "-";
    if (row.IsNull("bar")) row["bar"] = "-";
    row["date"] = ((DateTime)row["date"]).Date;
}

Alternatively, you could create new columns in the table, using expressions to autogenerate content:

table.Columns.Add("foo_dash", typeof(string), "IsNull(foo, '-')");
table.Columns.Add("bar_dash", typeof(string), "IsNull(bar, '-')");

(I don't know the date functions in ADO.NET so you will have to figure the last one out yourself.)

You have tagged your post ASP.NET, so I guess it is reasonable to assume that you are going to bind your DataTable to some multi-record data control (GridView, Repeater, etc). If this is the case, it might be better to do the transformations during databinding instead:

protected void theGrid_OnRowDataBound(object sender, GridViewRowEventArgs e)
{
    var data = e.DataItem as DataRowView;
    if (data != null)
    {
        if (data.Row.IsNull("foo")) e.Row.Cells[0] = "-";
        if (data.Row.IsNull("bar")) e.Row.Cells[0] = "-";
    }
}

While this seems to require a bit more code, it also gives you more flexibility. Example:

if (data.Row.IsNull("importantField")) e.Row.CssClass = "error";

In a GridView, the date can be formatted using a DataFormatString in the column declaration:

<asp:BoundField DataField="data" DataFormatString="{0:d}" />

Similar when databinding a Repeater:

<%# Eval("date", "{0:d}") %>

One way to do this would be to go through the table, building a HashSet<string> that contains the combined column values you're interested in. If you try to add a string that's already there, then you have a duplicate row. Something like:

HashSet<string> ScannedRecords = new HashSet<string>();

foreach (var row in dtCSV.Rows)
{
    // Build a string that contains the combined column values
    StringBuilder sb = new StringBuilder();
    foreach (string col in columns)
    {
        sb.AppendFormat("[{0}={1}]", col, row[col].ToString());
    }

    // Try to add the string to the HashSet.
    // If Add returns false, then there is a prior record with the same values 
    if (!ScannedRecords.Add(sb.ToString())
    {
        // This record is a duplicate.
    }
}

That should be very fast.