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Thanks for @RobH's answer and on top of it Micro-Level Code Optimization,

Thanks for @RobH's answer and on top of it Micro-Level Code Optimization, such as:

• use String Constant
• in String.Replace use char datatype instead of string datatype
• knownAccountNumbers value has been added while retrieving itself

Thanks for @RobH's answer and on top of it,

var knownAccountNumbers = new HashSet<string>();

//// If AccountData DataTable used only for condition then 
//// Delete Code Related to AccountData DataTable and Use knownAccountNumbers HashSet alone.
DataTable accountDt = dataStructure.Tables["AccountData"];

while (readFile.Read())
{
    string accountNumber = readFile.GetString(0).Trim();

    knownAccountNumbers.Add(accountNumber);
 
    DataRow dr = accountDt.NewRow();

    dr[AppConst.AccountNumber] = accountNumber;
    dr[AppConst.LegalStatus] = readFile.GetString(1);

    accountDt.Rows.Add(dr);
}

/* ----------------------------------------------------------------------------------------------- */

foreach (DataRow row in dt.Rows)
{
    //// For single char string replace use data type as char instead of string.
    //// Gives more perfromance
    //// var accountNumber = row[0].ToString().Replace(AppConst.BackSlashChar, AppConst.EmptyChar);
    var accountNumber = row[0].ToString().Replace(AppConst.BackSlash, string.Empty);

    //// Use Constant String will not create unnecessary memory allocation.
    row[12] = knownAccountNumbers.Contains(accountNumber) ? AppConst.Yes : AppConst.No;
}

/*------------------------------------------------------------------------------------------------ */

public static class AppConst
{
    public const string Yes      = "Y";
    public const string No       = "N";
    //// public const char   BackSlashChar = '\"';
    //// public const char   EmptyChar = '\0';
    public const string BackSlash = "\"";

    public const string AccountNumber = "AccountNumber";
    public const string LegalStatus   = "LegalStatus";
}

Thanks for @RobH's answer and on top of it Micro-Level Code Optimization,

var knownAccountNumbers = new HashSet<string>();

//// If AccountData DataTable used only for condition then 
//// Delete Code Related to AccountData DataTable and Use knownAccountNumbers HashSet alone.
DataTable accountDt = dataStructure.Tables["AccountData"];

while (readFile.Read())
{
    string accountNumber = readFile.GetString(0).Trim();

    knownAccountNumbers.Add(accountNumber);
 
    DataRow dr = accountDt.NewRow();

    dr[AppConst.AccountNumber] = accountNumber;
    dr[AppConst.LegalStatus] = readFile.GetString(1);

    accountDt.Rows.Add(dr);
}

/* ----------------------------------------------------------------------------------------------- */

foreach (DataRow row in dt.Rows)
{
    //// For single char string replace use data type as char instead of string.
    //// Gives more perfromance
    //// var accountNumber = row[0].ToString().Replace(AppConst.BackSlashChar, AppConst.EmptyChar);
    var accountNumber = row[0].ToString().Replace(AppConst.BackSlash, string.Empty);

    //// Use Constant String will not create unnecessary memory allocation.
    row[12] = knownAccountNumbers.Contains(accountNumber) ? AppConst.Yes : AppConst.No;
}

/*------------------------------------------------------------------------------------------------ */

public static class AppConst
{
    public const string Yes      = "Y";
    public const string No       = "N";
    //// public const char   BackSlashChar = '\"';
    //// public const char   EmptyChar = '\0';
    public const string BackSlash = "\"";

    public const string AccountNumber = "AccountNumber";
    public const string LegalStatus   = "LegalStatus";
}

Thanks for @RobH Answer and On Top It,

var knownAccountNumbers = new HashSet<string>();

//// If AccountData DataTable used only for condition then 
//// Delete Code Related to AccountData DataTable and Use knownAccountNumbers HashSet alone.
DataTable accountDt = dataStructure.Tables["AccountData"];

while (readFile.Read())
{
    string accountNumber = readFile.GetString(0).Trim();

    knownAccountNumbers.Add(accountNumber);
 
    DataRow dr = accountDt.NewRow();

    dr[AppConst.AccountNumber] = accountNumber;
    dr[AppConst.LegalStatus] = readFile.GetString(1);

    accountDt.Rows.Add(dr);
}

/* ----------------------------------------------------------------------------------------------- */

foreach (DataRow row in dt.Rows)
{
    //// For single char string replace use data type as char instead of string.
    //// Gives more perfromance
    //// var accountNumber = row[0].ToString().Replace(AppConst.BackSlashChar, AppConst.EmptyChar);
    var accountNumber = row[0].ToString().Replace(AppConst.BackSlash, string.Empty);

    //// Use Constant String will not create unnecessary memory allocation.
    row[12] = knownAccountNumbers.Contains(accountNumber) ? AppConst.Yes : AppConst.No;
}

/*------------------------------------------------------------------------------------------------ */

public static class AppConst
{
    public const string Yes      = "Y";
    public const string No       = "N";
    //// public const char   BackSlashChar = '\"';
    //// public const char   EmptyChar = '\0';
    public const string BackSlash = "\"";

    public const string AccountNumber = "AccountNumber";
    public const string LegalStatus   = "LegalStatus";
}

Thanks for @RobH's answer and on top of it,

var knownAccountNumbers = new HashSet<string>();

//// If AccountData DataTable used only for condition then 
//// Delete Code Related to AccountData DataTable and Use knownAccountNumbers HashSet alone.
DataTable accountDt = dataStructure.Tables["AccountData"];

while (readFile.Read())
{
    string accountNumber = readFile.GetString(0).Trim();

    knownAccountNumbers.Add(accountNumber);
 
    DataRow dr = accountDt.NewRow();

    dr[AppConst.AccountNumber] = accountNumber;
    dr[AppConst.LegalStatus] = readFile.GetString(1);

    accountDt.Rows.Add(dr);
}

/* ----------------------------------------------------------------------------------------------- */

foreach (DataRow row in dt.Rows)
{
    //// For single char string replace use data type as char instead of string.
    //// Gives more perfromance
    //// var accountNumber = row[0].ToString().Replace(AppConst.BackSlashChar, AppConst.EmptyChar);
    var accountNumber = row[0].ToString().Replace(AppConst.BackSlash, string.Empty);

    //// Use Constant String will not create unnecessary memory allocation.
    row[12] = knownAccountNumbers.Contains(accountNumber) ? AppConst.Yes : AppConst.No;
}

/*------------------------------------------------------------------------------------------------ */

public static class AppConst
{
    public const string Yes      = "Y";
    public const string No       = "N";
    //// public const char   BackSlashChar = '\"';
    //// public const char   EmptyChar = '\0';
    public const string BackSlash = "\"";

    public const string AccountNumber = "AccountNumber";
    public const string LegalStatus   = "LegalStatus";
}