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Writing Verifiably Type-Safe Code

Type-safe code is code that accesses types only in well-defined, allowable ways. For example, given a valid object reference, type-safe code can access memory at fixed offsets corresponding to actual field members. However, if the code accesses memory at arbitrary offsets outside the range of memory that belongs to that object's publicly exposed fields, it is not type-safe.

JIT compilation performs a process called verification that examines code and attempts to determine whether the code is type-safe. Code that is proven during verification to be type-safe is called verifiably type-safe code. Code can be type-safe, yet not be verifiably type-safe, due to the limitations of the verification process or of the compiler. Not all languages are type-safe, and some language compilers, such as Microsoft Visual C++, cannot generate verifiably type-safe managed code. To determine whether the language compiler you use generates verifiably type-safe code, consult the compiler's documentation. If you use a language compiler that generates verifiably type-safe code only when you avoid certain language constructs, you might want to use the Windows Software Development Kit (SDK) PEVerify tool to determine whether your code is verifiably type-safe.

Code that is not verifiably type-safe can attempt to execute if security policy allows the code to bypass verification. However, because type safety is an essential part of the runtime's mechanism for isolating assemblies, security cannot be reliably enforced if code violates the rules of type safety. By default, code that is not type-safe is allowed to run only if it originates from the local computer. Therefore, mobile code should be type-safe.