C# - How To Use Lambda Expression To Select Different Fields From Field Names

I need to get two fields from a database table (retrieved using linq-to-sql), one field is a datetime (and is a fixed field) and the other is always a decimal, but the field can be different.

The table holds currency data which is processed twice a day and in different currencies so could have fields such as AM_USD, PM_USD, AM_EUR etc. And I need to get data such as a list of the date against PM_USD or the date against AM_EUR.

I would like to be able to call the data using a lambda expression for example (this is a stripped out example):

data = TableData.Select(x=>new {x.DateTimeAdded, x.[**field name as string**]});

I have been trying to write a function to do this, and am failing dismally.

The closest I have managed is:

private Func<TableData, KeyValuePair<DateTime, decimal>> CreateSelect(string FieldName)
{
var parameterExp = Expression.Parameter(typeof(TableData), "sel");
var dateParameter = Expression.Parameter(typeof(DateTime), "DateTimeAdded");
var fieldParameter = Expression.Parameter(typeof(decimal), FieldName);
ConstructorInfo constructorInfo = typeof(KeyValuePair<DateTime, decimal>).GetConstructor(new[] { typeof(DateTime), typeof(decimal) });
NewExpression constructExpression = Expression.New(constructorInfo, new ParameterExpression[] { dateParameter, fieldParameter});
var lambda = Expression.Lambda<Func<TableData, KeyValuePair<DateTime, decimal>>>( constructExpression, parameterExp);
return lambda.Compile();
}

Which fails with "System.InvalidOperationException: Lambda Parameter not in scope".

I'm sure I missing something obvious, or going about it wrong way.