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# sendCommand(apdu:completionHandler:)

Sends an application protocol data unit (APDU) to the tag and receives a response APDU.

```
func sendCommand(apdu: NFCISO7816APDU, completionHandler: @escaping @Sendable (Data, UInt8, UInt8, (any Error)?) -> Void)
```

## Parameters

`apdu`

An application protocol data unit to send to the tag.

`completionHandler`

A handler that the reader session invokes after the operation completes. The handler has the following parameters:

- responseData: The response data, which may be empty even if the operation completes successfully.
- sw1: The SW1 command-processing status byte. This value is always valid.
- sw2: The SW2 command-processing status byte. This value is always valid.
- error: `nil` when the operation completes successfully; otherwise, an <doc://com.apple.documentation/documentation/Foundation/NSError> object when there’s a communication issue with the tag.

## Discussion

When you send a `SELECT` command with a [`p1Parameter`](/documentation/CoreNFC/NFCISO7816APDU/p1Parameter) value of `0x04`, your app must support one of the applications listed in the <doc://com.apple.documentation/documentation/BundleResources/Entitlements/com.apple.developer.nfc.readersession.iso7816.select-identifiers> property list key. Otherwise, the `completionHandler` receives an [`NFCReaderError.Code.readerErrorSecurityViolation`](/documentation/CoreNFC/NFCReaderError-swift.struct/Code/readerErrorSecurityViolation) error.

The session calls `completionHandler` on the dispatch queue that you provided when creating the [`NFCTagReaderSession`](/documentation/CoreNFC/NFCTagReaderSession) object.

---

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