You will notice GetExcelSheets function being called on the click of Upload Button. As the name suggests the function reads the names of all the sheets present in the Excel Workbook and binds the result to DropDownList.

The complete function is given below.

C#

private void GetExcelSheets(string FilePath, string Extension, string isHDR)
{
    string conStr = "";
    switch (Extension)
    {
        case ".xls": //Excel 97-03
            conStr = ConfigurationManager.ConnectionStrings["Excel03ConString"].ConnectionString;
            break;
        case ".xlsx": //Excel 07
            conStr = ConfigurationManager.ConnectionStrings["Excel07ConString"].ConnectionString;
            break;
    }

    //Get the Sheets in Excel WorkBoo
    conStr = String.Format(conStr, FilePath, isHDR);
    OleDbConnection connExcel = new OleDbConnection(conStr);
    OleDbCommand cmdExcel = new OleDbCommand();
    OleDbDataAdapter oda = new OleDbDataAdapter();
    cmdExcel.Connection = connExcel;
    connExcel.Open();

    //Bind the Sheets to DropDownList

    ddlSheets.Items.Clear();
    ddlSheets.Items.Add(new ListItem("--Select Sheet--", ""));
    ddlSheets.DataSource = connExcel.GetOleDbSchemaTable(OleDbSchemaGuid.Tables, null);
    ddlSheets.DataTextField = "TABLE_NAME";
    ddlSheets.DataValueField = "TABLE_NAME";
    ddlSheets.DataBind();
    connExcel.Close();
    txtTable.Text = "";
    lblFileName.Text = Path.GetFileName(FilePath);
    Panel2.Visible = true;
    Panel1.Visible = false;
}

VB.Net

Private Sub GetExcelSheets(ByVal FilePath As String, ByVal Extension As String, ByVal isHDR As String)
    Dim conStr As String = ""
    Select Case Extension
        Case ".xls"
            'Excel 97-03
            conStr = ConfigurationManager.ConnectionStrings("Excel03ConString").ConnectionString()
            Exit Select
        Case ".xlsx"
            'Excel 07
            conStr = ConfigurationManager.ConnectionStrings("Excel07ConString").ConnectionString()
            Exit Select
    End Select

    'Get the Sheets in Excel WorkBoo
    conStr = String.Format(conStr, FilePath, isHDR)
    Dim connExcel As New OleDbConnection(conStr)
    Dim cmdExcel As New OleDbCommand()
    Dim oda As New OleDbDataAdapter()
    cmdExcel.Connection = connExcel
    connExcel.Open()

    'Bind the Sheets to DropDownList
    ddlSheets.Items.Clear()
    ddlSheets.Items.Add(New ListItem("--Select Sheet--", ""))
    ddlSheets.DataSource = connExcel.GetOleDbSchemaTable(OleDbSchemaGuid.Tables, Nothing)
    ddlSheets.DataTextField = "TABLE_NAME"
    ddlSheets.DataValueField = "TABLE_NAME"
    ddlSheets.DataBind()
    connExcel.Close()
    txtTable.Text = ""
    lblFileName.Text = Path.GetFileName(FilePath)
    Panel2.Visible = True
    Panel1.Visible = False
End Sub

Ref: 

Read and Import Excel Sheet into SQL Server Database in ASP.Net