This sample code is for Solenoid Door Lock with Relay on Arduino.

Solenoid_Relay.ino

#define BUTTON  2     // the number of the pushbutton pin
#define RELAY   3     // the number of the relay pin

// variables will change:
int buttonState = 0;         // variable for reading the pushbutton status

void setup() {
  // initialize the LED pin as an output:
  pinMode(RELAY, OUTPUT);
  // initialize the pushbutton pin as an input:
  pinMode(BUTTON, INPUT_PULLUP);
}

void loop() {
  // read the state of the pushbutton value:
  buttonState = digitalRead(BUTTON);

  // check if the pushbutton is pressed.
  if (buttonState == HIGH) {
    // turn LED on:
    digitalWrite(RELAY, HIGH);
  } else {
    // turn LED off:
    digitalWrite(RELAY, LOW);
  }
}