SharpDevelop was stopped a few years ago. While it is a great alternative to Visual Studio it cannot use more recent versions of the C# toolset/compiler.
These are defined in the .csproj project settings file. You can open this file in a text editor and set the ToolsVersion="4.0", this is the last one for SharpDevelop.
I tried with SB-Prime changing the 3 projects from 15 to 4 and loading in SharpDevelop.
Also I changed TargetFrameworkVersion from 4.8 to to the latest 4.5.2 I have.
There are then some errors because I have used some syntax present in toolset 15, not present in version 4. Some of these I could fix, but couldn't get SB-Prime to fully compile, mainly due to some of the external packages and other referenced dlls that use .Net 4.8 functionality.
It is going to be hard to to just port code backwards. I suggest building you program from the scratch to start with, you will learn more.
These are defined in the .csproj project settings file. You can open this file in a text editor and set the ToolsVersion="4.0", this is the last one for SharpDevelop.
I tried with SB-Prime changing the 3 projects from 15 to 4 and loading in SharpDevelop.
Also I changed TargetFrameworkVersion from 4.8 to to the latest 4.5.2 I have.
There are then some errors because I have used some syntax present in toolset 15, not present in version 4. Some of these I could fix, but couldn't get SB-Prime to fully compile, mainly due to some of the external packages and other referenced dlls that use .Net 4.8 functionality.
It is going to be hard to to just port code backwards. I suggest building you program from the scratch to start with, you will learn more.