how to use single variable instead of using multiple variables in c#.net

You could write a utility method:

static decimal TryParse(
  string value, decimal @default = 0M)
{
    decimal ret;
    return decimal.TryParse(value, out ret) ? ret : @default;
}

and use that:

drTmp["empl_Basic"] = TryParse(txtBasicSalary.Text);
drTmp["empl_DA"] = TryParse(txtDA.Text);
drTmp["empl_HRA"] = TryParse(txtHRA.Text);

the usage below also allows for non-zero handling of invalid data:

drTmp["empl_HRA"] = TryParse(txtHRA.Text, 6.5M);

You can use a method if you want, for example as extension method:

public static Decimal? TryGetDecimal(this string item)
{
    Decimal d;
    bool success = Decimal.TryParse(item, out d);
    return success ? (Decimal?)d : (Decimal?)null;
}

Now you don't need explicit variable declarations at all, just use the return value:

drTmp["empl_Basic"] = txtBasicSalary.Text.TryGetDecimal() ?? 0;
drTmp["empl_DA"] = txtDA.Text.TryGetDecimal() ?? 0;

Yes, you can. I once came across your issue and implemented an extension for the Textbox control. It's a generic method that can convert do Decimal, int, anything that is a value type.

public static T? GetTextOrNullStruct<T>(this TextBox txt, GlobalSist.Common.Utils.ParserCondition? parserCondition, bool throwOnValidation)
    where T : struct, IComparable<T>
{
    return GlobalSist.Common.Utils.Parsers.ConvertStruct<T>(
        GetTextOrNull(txt), parserCondition, throwOnValidation);
}


public static T? ConvertStruct<T>(IConvertible value, ParserCondition? parserCondition, bool throwOnValidation)
    where T : struct, IComparable<T>
{
    try
    {
        if ((value == null) ||
            (value is string && string.IsNullOrEmpty((string)value)))
        {
            if (throwOnValidation)
                throw new ArgumentNullException("value");
            else
                return null;
        }
        return Parsers.Convert<T>(value, parserCondition, true);
    }
    catch (ArgumentOutOfRangeException)
    {
        if (throwOnValidation)
            throw;
        else
            return null;
    }
}

And here it is the method that converts to a string

public static T Convert<T>(IConvertible value, ParserCondition? parserCondition, bool throwOnValidation)
            where T : IComparable<T>
        {
            T convertedValue;
            try
            {
                convertedValue = (T)value.ToType(typeof(T), null);
            }
            catch (Exception ex)
            {
                if (throwOnValidation)
                    throw new ArgumentOutOfRangeException(ex.Message, ex);
                else
                    return default(T);
            }
            return ValidateParserCondition<T>(convertedValue, parserCondition, throwOnValidation);
        }

edit: Forgot to paste the last called method

private static T ValidateParserCondition<T>(T value, ParserCondition? parserCondition, bool throwOnValidation)
    where T : IComparable<T>
{
    if (parserCondition == null)
        return value;
    else
    {
        int comparingToZero = value.CompareTo(default(T));
        switch (parserCondition.Value)
        {
            case ParserCondition.GreaterOrEqualToZero:
                if (comparingToZero >= 0)
                    return value;
                break;
            case ParserCondition.GreaterThanZero:
                if (comparingToZero > 0)
                    return value;
                break;
            case ParserCondition.LessOrEqualToZero:
                if (comparingToZero <= 0)
                    return value;
                break;
            case ParserCondition.LessThanZero:
                if (comparingToZero < 0)
                    return value;
                break;
            default:
                throw new NotImplementedException("ParserCondition at ValidateParserCondition");
        }
        if (throwOnValidation)
            throw new ArgumentOutOfRangeException(
                string.Format("value {0} not in accordance with ParserCondition {1}",
                value, parserCondition.Value.ToString()));
        else
            return default(T);
    }

}

And the enum declaration is:

public enum ParserCondition
{
    /// <summary>
    /// &gt;=0
    /// </summary>
    GreaterOrEqualToZero,
    /// <summary>
    /// &gt;0
    /// </summary>
    GreaterThanZero,
    /// <summary>
    /// &lt;=0
    /// </summary>
    LessOrEqualToZero,
    /// <summary>
    /// &lt;0
    /// </summary>
    LessThanZero,

}