What is SOAP?

SOAP (Simple Object Access Protocol) is a protocol for exchanging structured messages in distributed environments. It is commonly used for invoking remote procedures (RPC) between applications via web services.

Main features of SOAP:

• Based on XML: SOAP messages are formatted in XML.
• Transport-independent: It can use various protocols for message transmission (HTTP, SMTP, TCP, etc.), although HTTP is the most common.
• Extensible: Allows adding custom headers for metadata, security, etc.
• Strictly defined: Messages have a clear structure:
  • Envelope: The root element defining the message.
  • Header: (Optional) Contains metadata not related to the payload.
  • Body: Contains the actual message data (method call, service response).
  • Fault: (Optional in Body) Contains error information.

Example of a SOAP message structure (request):

<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:ws="http://example.com/ws">
   <soapenv:Header/>
   <soapenv:Body>
      <ws:GetUserInfo>
         <ws:UserId>123</ws:UserId>
      </ws:GetUserInfo>
   </soapenv:Body>
</soapenv:Envelope>

Example of a SOAP message structure (response):

<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:ws="http://example.com/ws">
   <soapenv:Header/>
   <soapenv:Body>
      <ws:GetUserInfoResponse>
         <ws:UserName>John Doe</ws:UserName>
         <ws:UserEmail>john.doe@example.com</ws:UserEmail>
      </ws:GetUserInfoResponse>
   </soapenv:Body>
</soapenv:Envelope>

SOAP is often used together with WSDL (Web Services Description Language) to describe available web service operations and XSD (XML Schema Definition) to describe the data structure transmitted in messages.

In C#, working with SOAP services typically involves tools that generate proxy classes based on WSDL (such as adding "Service Reference" or "Web Reference" in Visual Studio). These proxy classes abstract the details of SOAP message formation and parsing.

SOAP is a mature technology widely used in enterprise systems, but recently it has often been replaced by more lightweight REST services for public APIs.