Object reference not set to an instance of an object.

this line?

 client.SendMessage(textMsg);

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thank you for the reply, yes so how can i solve it?

can u debug the code?
is the SmppClient client instance is null?

fro where i can know if its null or not?

if you can debug the code and reproduce the exception every time then simply check if client is null.
also, u can add code which checks if the client is null.
i'd also use try/catch to detect what and where the error occur:

protected void Page_Load(object sender, EventArgs e)
        {try{
            SmppClient client = new SmppClient();
            SmppConnectionProperties properties = client.Properties;
            properties.SystemID = "username";
            properties.Password = "password";
            properties.Port = 3333; 
            properties.Host = "xxx.xxx.xxx.xxx";
            properties.SystemType = "";
            properties.DefaultServiceType = "xxx";
      
            client.AutoReconnectDelay = 3000;

            client.KeepAliveInterval = 15000;

            client.Start();

            //--send SMS
            TextMessage textMsg = new TextMessage();
            textMsg.Text = "Hello, this is my message!";
            textMsg.DestinationAddress = "99999999";
            textMsg.SourceAddress = "888888888";
          if(client != null){
            client.SendMessage(textMsg);
}else{
//log error to file or raise error
}
}catch(exception ex){
//log exception to file or raise error
}
        }

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i tried your segusstion and i am getting the following error in catch

System.NullReferenceException: Object reference not set to an instance of object
at JamaaTech.Smpp.Net.Client.SmppClient.SendMessage(ShortMessage masage)

which line?

the error shows in line

client.SendMessage(textMsg)

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which mean its pass the if statement

try replace textMsg with string.Empty, is it still throwing exception?

yes still

the problem is in the implementation of SendMessage function.
do u have the source of SmppClient ?

SmppClient is dll file i download it from

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