How to dynamically generate Lambda expression in c#

Question

I would like to do the following;

    public IEnumerable<SalesRegister> GetSalesRegister()
    {
        return _client.GetAllSalesRegisters().OrderBy(x => x.CompanyName);
    }

by doing something like;

    public IEnumerable<SalesRegister> GetSalesRegister(string sortBy)
    {
        return _client.GetAllSalesRegisters().OrderBy(x => sortBy);
    }

for this I have tried the following, but didn't work

    public IEnumerable<SalesRegister> GetSalesRegister(string sortBy)
    {
        var type = typeof(SalesRegister);
        var param = Expression.Parameter(type, "x");
        var len = Expression.PropertyOrField(param, sortBy);
        return _client.GetAllSalesRegisters().OrderBy(x => len );
    }

I am not sure if I can do it in this way, can somebody help me please?

brainless coder · Accepted Answer · 2016-02-15 00:40:39Z

3

Try this -

public IEnumerable<SalesRegister> GetSalesRegister(Expression<Func<SalesRegister, object>> sortByExp)
{
    return _client.GetAllSalesRegisters().OrderBy(sortByExp.Compile());
}

You can call it like this -

GetSalesRegister(x => x.CompanyName);

To pass a lambda expression you need to have the type as Expression<Func<T, TR>> where T is base class and TR is the return type of the expression

Or, if you only have the string name try building the expression like this -

public IEnumerable<SalesRegister> GetSalesRegister(string sortBy)
{
    var param = Expression.Parameter(typeof (SalesRegister), "x");
    var prop = typeof (SalesRegister).GetProperty(sortBy);
    var objectFuncType = typeof (Func<,>).MakeGenericType(typeof (SalesRegister), prop.PropertyType);
    var propExp = Expression.PropertyOrField(param, sortBy);
    var exp = Expression.Lambda(objectFuncType, propExp, param);
    var converted = Expression.Convert(exp.Body, typeof (object)); // only needed if you are passing in no referene types like int, double, etc as parameters, otherwise ignore this line and use 'exp' in place of 'converted' in the next line
    var sortByExp= Expression.Lambda<Func<SalesRegister, object>>(converted, exp.Parameters);

    return _client.GetAllSalesRegisters().OrderBy(sortByExp.Compile());
}

You can call it like this -

GetSalesRegister("CompanyName");

edited Feb 15, 2016 at 0:40

answered Feb 14, 2016 at 23:55

brainless coder

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5 Comments

Learner Over a year ago

what is TDomain here?

brainless coder Over a year ago

That is a typo, this is a copied code from one of my implementations. Missed to replace with the actual type. use SalesRegister for your implementation.

Learner Over a year ago

I am getting compile time error now - The type arguments for method 'IOrderedEnumerable<TSource> System.Linq.Enumerable.OrderBy<Tsource,TKey(this IEnumerable<TSource>,Func<TSource,TKey>)>' cannot be inferred from the usage. Try specifying the type arguments explicitly. in the last line of the GetSalesRegister Method.

brainless coder Over a year ago

Add the type argument for the OrderBy like this - OrderBy<SalesRegister>(...)

brainless coder Over a year ago

Let us continue this discussion in chat.

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