### Problem

The problem is the actual [lambda expression][1] itself `() => $"Hello, {name}"` does not actually have a type (*yet*).  

It is only typed once it's cast to either:

* A [delegate][2] of type [`Func`][3] or [`Action`][4]
* Or an [expression tree][5] of type [`Expression`][8] (which can then be compiled to a delegate)

### Solution in C#10

[C#10][6] made several [lambda improvements][7] including an [inferred delegate type][9], meaning C# can now determine (infer) that the lambda is going to be used as a delegate function - so the original syntax is available upon **upgrade to C#10**:

```cs
var sayHello = () => $"Hello, {name}";
```

### Previous Solutions

If you can't upgrade to C#10, there are a couple ways you can explicitly type the lambda.

You can provide an explicit type instead of using `var`:

```cs
Func<string> sayHello = () => $"Hello, {name}";
```

Or you can use the func constructor to type the expression:

```cs
var sayHello = new Func<string>(() => $"Hello, {name}");
```

[**Demo in .NetFiddle**][12]

### Further Reading

* [delegate keyword vs. lambda notation][10]
* [How to return value with anonymous method?][11]

[1]: https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/operators/lambda-expressions 
[2]: https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/delegates/
[3]: https://learn.microsoft.com/en-us/dotnet/api/system.func-1?view=net-6.0
[4]: https://learn.microsoft.com/en-us/dotnet/api/system.action?view=net-6.0
[5]: https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/concepts/expression-trees/
[6]: https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-10
[7]: https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/csharp-10.0/lambda-improvements
[8]: https://learn.microsoft.com/en-us/dotnet/api/system.linq.expressions.expression?view=net-6.0
[9]: https://github.com/dotnet/csharpstandard/blob/standard-v6/standard/expressions.md#1116-anonymous-function-expressions
[10]: https://stackoverflow.com/q/299703/1366033
[11]: https://stackoverflow.com/q/10520892/1366033
[12]: https://dotnetfiddle.net/nhcZmb

I'm trying to write the following code:

```cs
var name = "Kyle";
var sayHello = () => $"Hello, {name}";
Console.WriteLine(sayHello());
```

But I get the error:

```none
Feature 'inferred delegate type' is not available in C# 9.0.
```

What does that mean and how can I fix it?

Answer

Problem

Solution in C#10

Previous Solutions

Further Reading