How to get bool result from async task function in C# – Error: Cannot implicitly convert type `void’ to `bool’

Statement 1. .Wait() has no return result. It is a void method, and therefore its result cannot be assigned to a variable.
You can use .Result which will wait until Task completes and return a result.

// Both are applicable to simple Tasks:
bool isValid = MyValidationFunction(jsonData).Result;

// does that same as

var task = MyValidationFunction(jsonData);
task.Wait();  
bool isValid = task.Result;

However, it is all valid for usual Tasks, but not for async/await functionality, because…

Statement 2. Do not mix up async and .Wait() – it still blocks the thread, killing the idea of async/await and negating all the performance improvement.

It also causes deadlock in WinForms, WPF, ASP.NET and other environments with SynchronizationContext.
Read more about it in this Stephen Cleary’s article or in these StackOverflow questions:

• An async/await example that causes a deadlock
• await works but calling task.Result hangs/deadlocks

Simple rule: if you use async, then you use await.

// That's how you do it with async/await:
public async bool GetJsonAndValidate()
{
     string jsonData = GetJson();
     bool isValid = await MyValidationFunction(jsonData); 
}

It will not block the thread and enable asynchronous behavior.