There are many sites available for the Automatic Watering System using Arduino controlled from PC or using android Mobile and coded in C#. Have you checked this, which is available on Codeproject:
Computer Controlled Watering System with C# or VB.NET[
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You can start searching on websites that are most profound in these kinds of projects written by many authors. Start searching with keywords, PC controlled automatic watering system using Arduino and C#.
You can also start from these websites which has great information.
Automatic Plant Watering System | Full Circuit With Source Code[
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Automatic Irrigation System Using Arduino : 5 Steps - Instructables[
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Automatic Plant Watering System Using Arduino[
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If you are facing some problem fixing errors in the code that you are working on, updating your question with the code you are working on will help you.