# Amazon GameLift Servers Realtime client API (C\$1) reference: Asynchronous callbacks
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Use this C\$1 Realtime Client API reference to help you prepare your multiplayer game for use with Amazon GameLift Servers Realtime deployed on Amazon GameLift Servers fleets. 
+ [Synchronous Actions](realtime-sdk-csharp-ref-actions.md)
+ Asynchronous Callbacks
+ [Data Types](realtime-sdk-csharp-ref-datatypes.md)

A game client needs to implement these callback methods to respond to events. The Realtime server invokes these callbacks to send game-related information to the game client. Callbacks for the same events can also be implemented with custom game logic in the Realtime server script. See [Script callbacks for Amazon GameLift Servers Realtime](realtime-script-callbacks.md).

Callback methods are defined in `ClientEvents.cs`.

## OnOpen()
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Invoked when the server process accepts the game client's connection request and opens a connection.

### Syntax
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```
public void OnOpen()
```

### Parameters
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This method takes no parameters.

### Return value
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This method does not return anything. 

## OnClose()
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Invoked when the server process terminates the connection with the game client, such as after a game session ends.

### Syntax
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```
public void OnClose()
```

### Parameters
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This method takes no parameters.

### Return value
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This method does not return anything. 

## OnError()
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Invoked when a failure occurs for a Realtime Client API request. This callback can be customized to handle a variety of connection errors.

### Syntax
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```
private void OnError(byte[] args)
```

### Parameters
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This method takes no parameters.

### Return value
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This method does not return anything. 

## OnDataReceived()
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Invoked when the game client receives a message from the Realtime server. This is the primary method by which messages and notifications are received by a game client.

### Syntax
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```
public void OnDataReceived(DataReceivedEventArgs dataReceivedEventArgs)
```

### Parameters
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**dataReceivedEventArgs**  
Information related to message activity.  
Type: [DataReceivedEventArgs](realtime-sdk-csharp-ref-datatypes.md#realtime-sdk-csharp-ref-datatypes-dataeventargs)  
Required: Yes

### Return value
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This method does not return anything. 

## OnGroupMembershipUpdated()
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Invoked when the membership for a group that the player belongs to has been updated. This callback is also invoked when a client calls `RequestGroupMembership`.

### Syntax
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```
public void OnGroupMembershipUpdated(GroupMembershipEventArgs groupMembershipEventArgs)
```

### Parameters
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**groupMembershipEventArgs**  
Information related to group membership activity.   
Type: [GroupMembershipEventArgs](realtime-sdk-csharp-ref-datatypes.md#realtime-sdk-csharp-ref-datatypes-groupeventargs)  
Required: Yes

### Return value
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This method does not return anything.