VB.net code question

Since the VB.net code is compiled,then why does a green squiggly line appear below a wrong line of code the very moment the cursor is moved away from it after writing?As in C++,when we use a variable without declaring it,we get to know the error only after giving the compile command.But in VB.net,the variable is underlined with a green line as soon as the cursor is moved away from it.Shouldn't this be seen only if the code was 'interpreted' and not 'compiled'??

The just in time compiler does a compile as the code is written and can identify errors before a compile is done at the project level. The project level compile is faster because much of the code is already compiled.