C# Language GetExpression Method


Example

public static Expression<Func<T, bool>> GetExpression<T>(IList<QueryFilter> filters)
{
    Expression exp = null;
    
    // Represents a named parameter expression. {parm => parm.Name.Equals()}, it is the param part
    // To create a ParameterExpression need the type of the entity that the query is against an a name
    // The type is possible to find with the generic T and the name is fixed parm
    ParameterExpression param = Expression.Parameter(typeof(T), "parm");

    // It is good parctice never trust in the client, so it is wise to validate.
    if (filters.Count == 0)
        return null;

    // The expression creation differ if there is one, two or more filters.    
    if (filters.Count != 1)
    {
        if (filters.Count == 2)
            // It is result from direct call.
            // For simplicity sake the private overloads will be explained in another example.
            exp = GetExpression<T>(param, filters[0], filters[1]);
        else
        {
            // As there is no method for more than two filters, 
            // I iterate through all the filters and put I in the query two at a time
            while (filters.Count > 0)
            {
                // Retreive the first two filters
                var f1 = filters[0];
                var f2 = filters[1];
                
                // To build a expression with a conditional AND operation that evaluates 
                // the second operand only if the first operand evaluates to true.
                // It needed to use the BinaryExpression a Expression derived class 
                // That has the AndAlso method that join two expression together
                exp = exp == null ? GetExpression<T>(param, filters[0], filters[1]) : Expression.AndAlso(exp, GetExpression<T>(param, filters[0], filters[1]));
                
                // Remove the two just used filters, for the method in the next iteration finds the next filters
                filters.Remove(f1);
                filters.Remove(f2);
                
                // If it is that last filter, add the last one and remove it
                if (filters.Count == 1)
                {
                    exp = Expression.AndAlso(exp, GetExpression<T>(param, filters[0]));
                    filters.RemoveAt(0);
                }
            }
        }
    }
    else
        // It is result from direct call.
        exp = GetExpression<T>(param, filters[0]);

           // converts the Expression into Lambda and retuns the query
    return Expression.Lambda<Func<T, bool>>(exp, param);
}