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How to get the first element of IEnumerable

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情书的邮戳
情书的邮戳 2020-12-29 06:34

Is there a better way getting the first element of IEnumerable type of this:

foreach (Image image in imgList)
{
     picture.Width = (short)image.Columns;
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  • 醉梦人生
    醉梦人生 (楼主)
    2020-12-29 07:00

    I had an issue where I changed my datasource from a bindingsource to an entity framework query.

    var query = dataSource as IQueryable;
var value = query.Where("prop = @0", value).Cast().SingleOrDefault();



    With entity framework this throw an exception `Unable to cast the type 'customer' to type 'object'. LINQ to Entities only supports casting EDM primitive or enumeration types.
    


    The class where my code was did not have a reference to the lib with the model so ...Cast was not possible.
    


    Anyway I used this approach
    


    var query = dataSource as IQueryable;
var targetType = query.GetType().GetGenericArguments()[0];
var value = query.Where("prop = @0", value).SingleOrDefault(targetType);

    


    in conjunction with an IEnumerable extension which uses reflection
    


        public static object SingleOrDefault(this IEnumerable enumerable, Type type)
    {
        var method = singleOrDefaultMethod.Value.MakeGenericMethod(new[] { type });
        return method.Invoke(null, new[] { enumerable });
    }

    private static Lazy singleOrDefaultMethod 
         = new Lazy(() =>
             typeof(Extensions).GetMethod(
                 "SingleOrDefault", BindingFlags.Static | BindingFlags.NonPublic));

    private static T SingleOrDefault(IEnumerable enumerable)
    {
        return enumerable.SingleOrDefault();
    }

    


    feel free to implement caching per type to improve performance.
    
    
    
             
    
                                                        
    
            
            
    
              
                
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