In Visual Studio, my Winforms program is not supporting interfaces. How can I make it support them?

Question

I've used Visual Studio in the past eg Visual Studio 2015, and prior, and not run into a problem like this.

I have Visual Studio 2019, and created a Winforms program and get this error

Default interface implementation is not available in c# 7.3. Please use language version 8.0 or higher

In the past, I used C# when it was c# 5.0 or earlier like C# v3 or C# v4, and I never ran into this.

I notice that Visual Studio 2019 offers two types of Winforms programs, .NET Framework and .NET Core

That error happens with the .NET Framework ones.

When I create a Winforms project picking .NET Core, then I don't get that error. I don't really know what the difference is between these two. But why does a Winforms program picked with .NET framework not support interfaces, but with .NET Core would?

I found one webpage said I need to install .net core 3, I did and the program (which was based on .NET framework) still didn't work.

Am I not meant to be creating Winforms programs based on .NET Framework? And only with .NET Core?

When I have a Winforms program based on .NET Framework, how am I supposed to get it to work?

I looked at downloading C# 8 I see this link https://devblogs.microsoft.com/dotnet/take-c-8-0-for-a-spin/ mentions .NET Core 3, though I think I already installed .NET Core 3. And the problem is with Winforms programs based on the .NET Framework, rather than ones based on .NET Core

Answer 1

You're trying to declare the following interface:

interface abcd
{
    static int abcd;
}

You're using .NET Framework, which defaults to C# 7.3. In C# 7.3, an interface cannot contain static members, or fields, so this is wrong twice. Before default interface implementations were added in C# 8.0, this would have been a different error; probably one telling you that interfaces cannot contain fields. After C# 8.0, however, it is possible to do this sort of thing as part of a default implementation, so now the error assumes you were trying to do that, and tells you what version of C# you need to enable in order to do so.

If you weren't trying to add a default implementation, then you can fix your code by making that field into a property, and making that property not static.

interface abcd
{
    int abcd { get; set; }
}

Answer 2

Error is pretty clear that it's only allowed in C# 8.0 or greater.

And C# 8.0 supports .net core

.net frameworks don't support version greater then C# 7.3.

Reference: https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/configure-language-version#defaults

Answer 3

info from vivek, hans and anax helped..

so there's two ways of creating a winforms application in VS 2019, one is .NET Core, one is .NET Framework (as we see looking at the image in the question)

Each is associated with a different version of the C# language.

https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/configure-language-version

Target framework    version C#      language version  
.NET Core           3.x              C# 8.0
.NET Core           2.x              C# 7.3
.NET Standard       2.1              C# 8.0
.NET Standard       2.0              C# 7.3
.NET Standard       1.x              C# 7.3
.NET Framework      all              C# 7.3

.NET Framework is the old name for the framework / name for an older framework, and .NET Core is more recent.

So it's probably better to create them choosing .NET Core

And if creating a .NET Core one, then the language used is C# 8.

Whereas when creating it with .NET Framework, then the language used is C# 7.3

Both don't allow instance variables in an interface but the more recent one (c# 8), which is used with .NET Core, does allow static variables in an interface.

VS 2015 used the old one, .NET Framework.

In VS 2015, I must have just used getters and setters eg

int abcd { get; set; }

Which of course you can still do with the newer one, the .NET Core one.