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c#linqexpression
Danicco
Danicco

Reputation: 1673

Converting a value from an expression to another type C#

I'm coding a custom query builder from expressions, and at some point I'm saving the value of an expression to my criteria class:

switch(expression.NodeType)
{
    case ExpressionType.Constant:
    {
        //Here there should only be raw values
        CriteriaClass newCriteria = new CriteriaClass();
        newCriteria.Value = expression; //Value is of 'object' type

        return newCriteria;
    }
}

And when I'm actually setting up the query, I have a list of all criterias and their values, which seem fine but... their types are all messed up. The problem is that my next step is converting properly the types of values to the specific DB format:

private string FormatWriteValue(object value, Type type)
{
    if (value == null) { return "NULL"; }

    if (value.GetType().IsEnum) { return ((int)value).ToString(); }

    switch(type.Name)
    {
        case "Boolean":
        case "bool":
            return ((bool)value) ? "1" : "0";
        case "Int32":
        case "int":
            return value.ToString();
        case "DateTime":
            return "CONVERT(DATETIME, '" + ((DateTime)value).ToString("dd/MM/yyyy hh:mm:ss") + "', 103)";
        default:
            return "'" + value.ToString().Replace("'", "''") + "'";
    }
}

Since the type is never one of the basic ones I've typed there, it always falls on the default case for strings. I've tried casting the value of the expression like this:

criteria.Value = (int)expression; //exception
criteria.Value = Convert.ChangeType(expression, expression.Type); //Type = Int32, but exception again
criteria.Value = Expression.Convert(expression, expression.Type); //Becomes Unary, no exception

I think for the last one to work I'd have to compile the expression, but I've read that it's costly and I'd like to keep this as light as possible.

How can I accomplish this?

Upvotes: 1

Views: 2715

Answers (1)

Kevin Avignon
Kevin Avignon

Reputation: 2903

When you create your LINQ query to create your data, verify what type of data it is. So for instance when you do confirm the type, you can parse a string into a DateTime struct like this :Parse string to DateTime in C#

The best thing to do with your LINQ.Expression type is this: Convert it into a string. Then convert that string dynamically into the type you need.

   var criteriaToBeConverted =  Expression.Call(
                     Expression.Convert(memberExpression, typeof(object)),
                     typeof(object).GetMethod("ToString"));

You can learn more about Expression here : http://msdn.microsoft.com/fr-fr/library/system.linq.expressions.expression.call(v=vs.110).aspx

The provided code comes from this actual answer : LINQ Expression Conversion / Concat from Int to string

When you have type as a string, create a method which reads the string and verifies what type the string represents and parse it. Then you return the value as an object.

Hope it helps you out !

If the answer helps you in anyway, check it as answer so others with the same problem can know what to do !

Upvotes: 1

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