The LINQ expression node type 'ArrayIndex' is not supported in LINQ to Entities

public List<string> GetpathsById(List<long> id)
{
    long[] aa = id.ToArray();
        long x;
    List<string> paths = new List<string>();
    for (int i = 0; i < id.Count; i++)
    {
        x = id[i];
        Presentation press = context.Presentations.Where(m => m.PresId == aa[i]).FirstOrDefault();
        paths.Add(press.FilePath);
    }
    return paths;
}

This code throws the following exception: The LINQ expression node type 'ArrayIndex' is not supported in LINQ to Entities.

However, if I supply x instead of aa[i] it works.

Why?

To fix this use a temporary variable:

var tmp = aa[i];
...
m => m.PresId == tmp

---

In your where clause you have

m => m.PresId == aa[i]

which is a way of expressing a lambda expression. When that is converted to an expression, then converted into a query on your database it finds the aa[i], which is an index into an array. i.e. it doesn't treat it as a constant. Since a translation of an indexer to your database language is impossible it gives the error.

Apparently, if you use an array index (aa[i]) inside an expression tree, it tries to convert that into an expression as well.

Just work around it by using a separate variable:

int presId = aa[i];
Presentation press = context.Presentations.Where(m => m.PresId == presId).FirstOrDefault();

 public List<string> GetpathsById(List<long> id)
{
long[] aa = id.ToArray();
    long x;
List<string> paths = new List<string>();
for (int i = 0; i < id.Count; i++)
{
    x = id[i];
    int temp = aa[i];
    Presentation press = context.Presentations.Where(m => m.PresId == temp).FirstOrDefault();
    paths.Add(press.FilePath);
}
return paths;
}

try this

It cannot be mapped to an SQL type or function.

You are aware you are mixing your list and array with each other. Everything you want doing in this code can be done simply using the list.

The following bit of code will do everything you need it to.

public List<string> GetpathsById(List<long> id) 
{ 
        long x; 
    List<string> paths = new List<string>(); 
    foreach(long aa in id) 
    { 
        Presentation press = context.Presentations.Where(m => m.PresId == aa).FirstOrDefault(); 
        paths.Add(press.FilePath); 
    } 
    return paths; 
} 

public List<string> GetpathsById(List<long> id) 
{ 
        long x; 
    List<string> paths = new List<string>(); 
    foreach(long aa in id) 
    { 
        Presentation press = context.Presentations.Where(m => m.PresId == aa).FirstOrDefault(); 
        paths.Add(press.FilePath); 
    } 
    return paths; 
} 

to

public IEnumerable<String> GetpathsById(List<long> id) 
{ 
    foreach(long item in id) 
        yield return = (context.Presentations.Where(m => m.PresId == item).FirstOrDefault()).FilePath
} 

"Short style", but not recomended if you write a lot of other function.

Can be simplified to avoid the error:

public List<string> GetpathsById(List<long> id)
{
    return context.Presentations.Where(x => id.Contains(x.PresId)).Select(x => x.FilePath).ToList();
}