Question

if we have :

dynamic x = "hello"; // Static type is dynamic, runtime type is string
var y = "hello"; // Static type is string, runtime type is string

Fine.

what about

dynamic x = *veryComplicatedRunTimeStructure_UnknownatCompileTime.*; 
var y=x; 

it DOES compile !

I have a problem with the second line here.

it is known that var is evaluated in compile time.

But in compile time it doesn’t have a clue about the type….

so…what is the compile type of the y?

Answer 1

When the type of the assignment expression is dynamic, so is the implicitly declared variable’s type. There’s nothing special about dynamic here – your code is equivalent to:

dynamic x = new { t = 3, y = 3 };
dynamic y = x;

From section 8.5.1 of the C# 4 spec:

In the context of a local variable declaration, the identifier var acts as a contextual keyword. When the local-variable-type is specified as var and no type named var is in scope, the declaration is an implicitly typed variable declaration, whose type is inferred from the type of the associated initializer expression.

[…]

In an implicitly typed local variable declaration, the type of the local variable being
declared is taken to be the same as the type of the expression used to initialize the variable.

So here, the expression is dynamic, and so is the variable…