[c#] IF Statement multiple conditions, same statement

Hey all, looking to reduce the code on my c# if statements as there are several repeating factors and was wondering if a trimmer solution is possible.

I currently have 2 if statements that need to carry out an identical statement, however the only variable is an extra condition on an if statement when a checkbox is not checked. Im just wondering if there is a way to make it one statement or make the condition string variable, heres the compressed version of the code:

if (checkbox.checked)
  {
    if (columnname != a && columnname != b && columnname != c)
    {
      "statement 1"
    }
  }
else
  {
    if (columnname != a && columnname != b && columnname != c 
        && columnname != A2)
    {
      "statement 1"
    }
  }

Its like I need to run an if statement within the conditions of an if statement if that makes sense, like this psuedo form:

if (columnname != a 
    && columnname != b 
    && columnname != c 
    && if(checkbox.checked{columnname != A2})

if (columnname != a && columnname != b && columnname != c 
        && (columnname != A2 || checkbox.checked))
    {
      "statement 1"
    }

I always try to factor out complex boolean expressions into meaningful variables (you could probably think of better names based on what these columns are used for):

bool notColumnsABC = (columnname != a && columnname != b && columnname != c);
bool notColumnA2OrBoxIsChecked = ( columnname != A2 || checkbox.checked );

if (   notColumnsABC 
    && notColumnA2OrBoxIsChecked )
  {
      "statement 1"
  }

I think agileguy has the correct answer, but I would like to add that for more difficult situations there are a couple of strategies I take to solve the problem. The first is to use a truth table. If you Google "truth table" you will run across some examples related directly to programming and computer science.

Another strategy I take is to use an anonymous function to encapsulate common logic between various conditions. Create it right before the if block, then use it where needed. This seems to create code that is more readable and maintainable.

var test = new char[] {a, b, c}.Contains(columnname));

if(test)
{
  "true statement"
}
else
{
   "false statement"
}

if (checkbox.checked && columnname != a && columnname != b && columnname != c)
    {
      "statement 1"
    }
else if (columnname != a && columnname != b && columnname != c 
        && columnname != A2)
    {
      "statement 1"
    }

is one way to simplify a little.

Isn't this the same:

if ((checkbox.checked || columnname != A2) && 
        columnname != a && columnname != b && columnname != c)
  {
      "statement 1"
  }

Pretty old question but check this for a more clustered way of checking conditions:

private bool IsColumn(string col, params string[] names) => names.Any(n => n == col);

usage:

private void CheckColumn()
{
     if(!IsColumn(ColName, "Column A", "Column B", "Column C"))
    {
     //not A B C column
    }

}

How about maybe something like this?

    var condCheck1 = new string[]{"a","b","c"};
    var condCheck2 = new string[]{"a","b","c","A2"}

    if(!condCheck1.Contains(columnName) && !checkbox.checked)
        //statement 1
    else if (!condCheck2.Contains(columnName))
        //statment 2

You could also do this if you think it's more clear:

if (columnname != a 
  && columnname != b 
  && columnname != c
{
   if (checkbox.checked || columnname != A2)
   {
      "statement 1"
   }
}