Is relying on && short-circuiting safe in .NET?

Yes. In C# && and || are short-circuiting and thus evaluates the right side only if the left side doesn’t already determine the result. The operators & and | on the other hand don’t short-circuit and always evaluate both sides.

The spec says:

The && and || operators are called the conditional logical operators. They are also called the “shortcircuiting” logical operators.
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The operation x && y corresponds to the operation x & y, except that y is evaluated only if x is true
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The operation x && y is evaluated as (bool)x ? (bool)y : false. In other words, x is first evaluated and converted to type bool. Then, if x is true, y is evaluated and converted to type bool, and this becomes the result of the operation. Otherwise, the result of the operation is false.

(C# Language Specification Version 4.0 – 7.12 Conditional logical operators)

One interesting property of && and || is that they are short circuiting even if they don’t operate on bools, but types where the user overloaded the operators & or | together with the true and false operator.

The operation x && y is evaluated as T.false((T)x) ? (T)x : T.&((T)x, y), where
T.false((T)x) is an invocation of the operator false declared in T, and T.&((T)x, y) is an invocation of the selected operator &. In addition, the value (T)x shall only be evaluated once.

In other words, x is first evaluated and converted to type T and operator false is invoked on the result to determine if x is definitely false.
Then, if x is definitely false, the result of the operation is the value previously computed for x converted to type T.
Otherwise, y is evaluated, and the selected operator & is invoked on the value previously computed for x converted to type T and the value computed for y to produce the result of the operation.

(C# Language Specification Version 4.0 – 7.12.2 User-defined conditional logical operators)