Long filepath / commandline not working in shell executin a .exe file

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C#6.0

When the file path itself is long, i get a commandline eith a file path over 100 characters long.
For some reason the convert.exe i'm working on doesn't work even if the paths are all Ok.

So how can i use a commandline "locally", if i just specify a path where it is?
Because if i go to command prompt, i can just run it specifying a source and target name, not the full path.
Example:
convert.exe file.jpg file.jpg
In code i'm using (have to use):
C:\path\path\path\convert.exe C:\path\path\path file.jpg C:\path\path\path file.jpg

Thanks in advance for any thoughts.

SSa

What I have tried:

It seems to work with shorter paths ok.
I tried to work differently, but i found i have to add full path.