Create Pre Computed Task Using Task FromResult

Task.FromResult method is useful when you perform an asynchronous operation that returns a Task<TResult> object, and the result of that Task<TResult> object is already computed.

The FromResult method returns a finished Task<TResult> object that holds the provided value as its Result property.

Samples

Convert a simple synchronous method to asynchronous method

Greeting is a simple method which returns a string

private static string Greeting() {
	return "Hello World";
}

public static void Main() 
{
	var message = Greeting();
	Console.WriteLine(message);
}

You can use Task.FromResult to convert into asynchronous method

private static Task<string> GreetingAsync() {
	return Task.FromResult("Hello World");
}

public static async Task Main() 
{
	var message = await GreetingAsync();
	Console.WriteLine(message);
}

Read from Cache if present or download

In the following example DownloadAsync method checks the cache for given address and uses FromResult method to produce a Task<TResult> object that holds the content at that address. Otherwise, DownloadStringAsync downloads the file from the web and adds the result to the cache.

private static readonly ConcurrentDictionary<string, string> cachedDownloads = new();
private static readonly HttpClient httpClient = new();

public static async Task Main()
{
	string[] urls = new[]
       {
           "https://docs.microsoft.com/aspnet/core",
           "https://docs.microsoft.com/dotnet",
           "https://docs.microsoft.com/dotnet/azure",
           "https://docs.microsoft.com/dotnet/machine-learning",
           "https://docs.microsoft.com/xamarin",
       };
	
	
	IEnumerable<Task<string>> downloads1 = urls.Select(DownloadAsync);
	await Task.WhenAll(downloads1);
	
	IEnumerable<Task<string>> downloads2 = urls.Select(DownloadAsync);
	await Task.WhenAll(downloads2);
}

public static Task<string> DownloadAsync(string address)
{
	if(cachedDownloads.TryGetValue(address, out string content)){
		Console.WriteLine($"Reading {address} data from cache");
		return Task.FromResult(content);
	}
	
	return Task.Run(async () => {
		var content = await httpClient.GetStringAsync(address);
		cachedDownloads.TryAdd(address, content);
		Console.WriteLine($"Reading {address} data from web");
		return content;
	});
}

Example