Why should I add _ to the start of var names?

[Superturkey77]

And if I should, when do I add the _s?

[andrew-lukasik]

I don’t think you “should” but “may want to” do that. It’s a matter of writing style - totally up to you (project owner/lead) choice.

I myself use this underscore prefix _ to visually and alphabetically distinct private fields from public ones, property accessors and local variables - to make them immediately distinguishable mid busy code blocks.

public int publicProperty { get; set; } = 1;
public int publicField = 1;
public int _privateField = 1;
public int PublicMethod ()
{
    int localVariable = 1.
    return publicField + publicProperty + _privateField + localVariable + PrivateMethod();
}
int PrivateMethod () => 1;

[vertxxyz]

Many people (myself included) also use the Microsoft coding conventions, which do include the _ prefix for private members. But unlike what Andrew posted, all public members are PascalCase.

Using Andrew’s example that would be:

public int PublicProperty { get; set; } = 1;
public int PublicField = 1;
private int _privateField = 1;

public int PublicMethod(int parameter)
{
    int localVariable = 1;
    return PublicField + PublicProperty + _privateField + localVariable + PrivateMethod();
}

private int PrivateMethod() => 1;

There are plenty of different styles out there, so find a pre-existing one (for C#/Unity) that you like and be aware that you may have to switch if you join someone else’s project.