C# Language Specification

Implicit dynamic conversions

An implicit dynamic conversion exists from an expression of type dynamic to any type T. The conversion is dynamically bound (§7.2.2), which means that an implicit conversion will be sought at run-time from the run-time type of the expression to T. If no conversion is found, a run-time exception is thrown.

Note that this implicit conversion seemingly violates the advice in the beginning of §6.1 that an implicit conversion should never cause an exception. However it is not the conversion itself, but the finding of the conversion that causes the exception. The risk of run-time exceptions is inherent in the use of dynamic binding. If dynamic binding of the conversion is not desired, the expression can be first converted to object, and then to the desired type.

The following example illustrates implicit dynamic conversions:

object o = “object”
dynamic d = “dynamic”;

string s1 = o; // Fails at compile-time – no conversion exists
string s2 = d; // Compiles and succeeds at run-time
int i = d; // Compiles but fails at run-time – no conversion exists

The assignments to s2 and i both employ implicit dynamic conversions, where the binding of the operations is suspended until run-time. At run-time, implicit conversions are sought from the run-time type of d – string – to the target type. A conversion is found to string but not to int.

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