C# Create Lambda Expression Dynamically

Question

I work with Dapper and I try to create auto-mapped method for inner join.
This is the example of the models:

public class User
{
    public long IdUser { get; set; }
    public string Email { get; set; }
} 

public class Page
{
    public long Id { get; set; }
    public string Name { get; set; }
    public long IdUserCreatedPage { get; set; }
    public User UserCreatedPage { get; set; }
}

This is the query:

SELECT * FROM "PAGE" INNER JOIN "USER" ON "PAGE"."IdUserCreatedPage" = "USER"."IdUser"

if I write code mannualy I will write this:

public List<Page> GetPage(IDbConnection dbConnection, string sql)
{
    return (List<Page>)dbConnection.Query<Page, User, Page>(sql, 
        (Page p, User u) =>
        {
            p.UserCreatedPage = u;
            return p;
        },
        splitOn: "IdUser").ToList();  
}

Now, what I want is create dynamically the Func<TFirst, TSecond, TOut> that I need for mapping the object.
Can someone help me? Thank you very much.

P.S. I know that in this case it does not make sense create it dynamically, but this it's only a simply version of all the project of auto-mapping.

This might be of some help : https://github.com/kahanu/System.Linq.Dynamic/wiki/Dynamic-Expressions — Richard Friend, Oct 10 '18 at 10:03
This is a question-and-answer site. If you have solved it, add and accept an answer. The practice of putting [SOLVED] into titles on forum sites is because those sites aren't designed for finding answers to questions, like this one is. — Jon Hanna, Oct 10 '18 at 12:02

score 8 · Accepted Answer · answered Oct 10 '18 at 12:06

SOLUTION

Finally I find the way to do what I want.
This is the code of the function that Generate Func<TFirst, TSecond, TOut>:

public static Func<TFirst, TSecond, TFirst> MappingDynamicFunc<TFirst, TSecond>()
{
    ParameterExpression paramFirst = Expression.Parameter(typeof(TFirst), "paramFirst");
    ParameterExpression paramSecond = Expression.Parameter(typeof(TSecond), "paramSecond");

    MemberExpression memberExpression = Expression.PropertyOrField(paramFirst, "UserCreatedPage");
    BinaryExpression assign = Expression.Assign(memberExpression, paramSecond);

    LabelTarget labelTarget = Expression.Label(typeof(TFirst));
    GotoExpression returnExpression = Expression.Return(labelTarget, paramFirst, typeof(TFirst));
    LabelExpression labelExpression = Expression.Label(labelTarget, Expression.Default(typeof(TFirst)));

    BlockExpression block = Expression.Block(
        assign,
        returnExpression,
        labelExpression
    );

    return Expression.Lambda<Func<TFirst, TSecond, TFirst>>(block, new ParameterExpression[] { paramFirst, paramSecond }).Compile();
}

And this is the "GetPage" method:

public List<Page> GetPage(IDbConnection dbConnection, string sql)
{
    return (List<Page>)dbConnection.Query<Page, User, Page>(sql, 
        MappingDynamicFunc<Page, User>(),
        splitOn: "IdUser").ToList();  
}

score 3 · Answer 2 · answered Oct 10 '18 at 12:08

Take a look at PredicateBuilder. http://www.albahari.com/nutshell/predicatebuilder.aspx

Here is some pseudo code

var predicate = PredicateBuilder.True<SomeClass>();

if (SomeCondition)
{
     var inner = PredicateBuilder.False<SomeClass>();
         inner = inner.Or(p => p.Category == "WhatEver");
         inner = inner.Or(p => p.Category == "");

     predicate = predicate.And(inner);
}

...

var result = MyIEnumerable<SomeClass>.AsQueryable()
                                     .Where(predicate)
                                     .FirstOrDefault();

C# Create Lambda Expression Dynamically

2 Answers2

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