They’re often used to detect when a door or drawer is open, which is why they have mounting tabs and screws.

Technical Description: 

• Normally open reed switch
• ABS enclosure
• Rated current: 100 mA max
• Rated voltage: 200 VDC max
• Distance: 15mm max

Limitations:
Depending on the sensor, the amount of resistance may vary, as well as the maximum amount of pressure that can be applied.

Skill:
Easy

Sample Sensor: 

Magnetic contact switch (door sensor)
https://www.adafruit.com/product/375

Sample Exercise:

https://archive.codeplex.com/?p=netduinohelpers#Hardware%2fPushButton.cs