I want to mention additional things and make a conclusion.
Commonly, As Otherones explained, This exception will raise when there is a problem with loading expected type (in an existing assembly) in runtime. More specific, Whenever there is loaded assembly that is stale (and not contains the requiring type). Or when an assembly with different version or build is already loaded that not contains the expected type.
Also, I must mention that It seems Rarely, But It is possible to be runtime limitation or a compiler bug (suppose compiler not negated about something and just compiled problematic code), But runtime throws System.TypeLoadException exception. Yes; this actually occurred for me!
An Example (witch occurred for me)
Consider an struct that defines a nullable field of its own type inside itself. Defining such the filed non-nullable, will take you a compile-time error and prevents you from build (Obviously This behavior has a logical reason). But what about nullable struct filed? Actually, nullable value-types is Nullable<T> at behind. As c# compiler not prevented me from defining in this way, I tried it and the project built. But I get runtime exception System.TypeLoadException: 'Could not load type 'SomeInfo' from assembly ... ', And It seems to be Problem in loading the part of my code (otherwise, we may say: the compiler not-truly performed compilation process at least) for me in my environment:
public struct SomeInfo
{
public SomeInfo? Parent { get; }
public string info { get; }
}
My environment specs:
(I checked and ensured that the compiled assembly is up to date And actually is newly compiled. Even, I throwed away all bin directory and refreshed the whole project. Even I created a new project in a fresh solution and tested, But such the struct generates the exception.)
It seems to be a runtime limitation (logically or technically) or bug or a same problem else, because Visual Studio not prevents me from compile And Also other newer parts of my codes (excluding this struct) are executing fine.
Changing the struct to a class, the compiled assembly contains and executes the type as well.
I have no any idea to explain in-detail why this behavior occurs in my environment, But I faced this situation.
Conclusion
Check the situations:
When a same (probably older) assembly already exists in GAC that is overriding the referenced assembly.
When re-compilation was need but not performed automatically (therefore the referenced assembly is not updated) and there is needs to perform build manually or fix solution build configuration.
When a custom tool or middleware or third-party executable (such as tests runner tool or IIS) loaded an older version of that assembly from cached things and there is need to cleaning somethings up or causing to reset somethings.
When an Unappropriated Configuration caused demanding a no longer existing type by a custom tool or middleware or a third-party executable that loads the assembly and there is need to update configuration or cleaning somethings up (such as removing an http handler in web.config file on IIS deployment as @Brian-Moeskau said)
When there is a logical or technical problem for runtime to execute the compiled assembly, (for example when compiler was compiled a problematic code that the in-using runtime cannot understand or execute it), as I faced.