Mbox, messagebox.show, window alert? Need a hand with a message box pop up, depending on the results of an If statement

Hello, basically what i need is a message box pop up that will either say 'this project exists, please select another' if the IF is true else 'project doesnt exist please carry on process'. I tried a few ways already but from looking on here I see Java script is favoured....unluckly my java knowledge is limited.

Any help is much appreciated thanks!

protected void btnProjSearch_Click(object sender, EventArgs e)
    {
 
        string OracleConnectionStringBuilder = "Data Source=TEST;User Id=***;Password=****";
 
        OracleConnection myConnection = new OracleConnection(OracleConnectionStringBuilder);
        myConnection.Open();
 
 
        string commandString2 = "SELECT PROJECTNO FROM P_AMASTER WHERE PROJECTNO like '" + txtProjNo.Text + "'";
 
    OracleCommand myCommand2 = new OracleCommand(commandString2, myConnection);
 
    OracleDataReader myReader2 = myCommand2.ExecuteReader();
    myReader2.Read();
 
        if (myReader2.HasRows)
        {
          
        }
        else
        {
 
        }
 
        
        myReader2.Close();   
        }
    }

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